GB2389578A - High temperature cement compositions for use downhole - Google Patents

Abstract

In a process to design high temperature cement slurries, temperature stable mineral phases (such as anorthite and wairakite) are selected that will form at the downhole temperature of the well. Aluminium modifiers and silica oxides are added to the dry cement so that the elemental composition of the blend corresponds to the mineral phases selected. The control of the particle size distribution and the relative amount of those minerals allow their addition at high concentration while leaving the slurry easily mixable and pumpable. The kinetics that affect how quickly the thermodynamic equilibrium of the targeted mineral phases formation is reached, can be influenced by adjusting the crystallinity and the particle sizes of the different solids.

Description

( HIGH TEMPERATURE CEMENT COMPOSITIONS

The present invention relates to compositions and methods for cementing operations conducted at high temperatures. In particular, the invention provides methods for 5 designing slurries for use in cementing hightemperature wells such as geothermal wells or oil and gas wells.

After drilling an oil well or the like, a casing or coiled tubing is lowered into the hole and cemented over all or part of its depth. In particular, cementing prevents exchange 10 of fluids between the different layers of formation traversed by the hole, prevents gas from rising in the annular space surrounding the casing or limits the ingress of water into the production well. Its principal purpose, of course, is to support the casing.

In the exploitation of geothermal wells or very high temperature oil and gas wells, 15 environmental conditions are much severe than in normal oil wells, with very high temperatures and highly aggressive brine. For safety and others reasons, the set cement sheath has to be durable in order to keep its integrity and to protect the casing from corrosion and isolate the fluids present in the different geological formations (Berra, M. et al,(l988) Behaviour of a cementing hydraulic binder under severe 20 geothermal conditions. Geothermics, Vol. 17, No. 5/6, pp 785-813).

In the vast majority of well cementing cases, a Portland cement is used as the hydraulic binder, a material that is composed of calcium silicates and calcium aluminosilicates. In the presence of water, those mineral phases hydrate and form 25 hydrated calcium silicate (CSH) and also portlandite (Ca(OH)2) and aluminium sulphate. At temperatures below about 1 00 C, CSH is a gel, and is largely responsible for the cohesion and mechanical strength of the set cement in the short to medium term. 30 When the temperature exceeds about 1 00 C, in the first days or months the majority of cements lose compressive strength and become permeable. Then they regain their strength and their permeability reduces. That phenomenon, known as retrogression, is due to metamorphosis of CSH gel to a highly crystalline phase known as alpha

dicalcium silicate, a-C2SH, which is much more dense than CSH with a contraction in volume and an increase in the porosity of the cement. This results in disintegration of the cement, which increases in rate as the porosity increases, rendering it more sensitive to chemical attack such as channelling of brine or circulation of carbon 5 dioxide.

To avoid the phenomenon of retrogression, it has been proposed to add silica in the form of sand or silica flour to compositions which have to withstand temperatures of more than 1 00 C. Adding 35% to 40% of silica (with respect to the weight of the 10 Portland cement) brings the CaO/SiO2 ratio (usually denoted C/S) to about 1.

With this modified composition, CSH gel reacts with the added silica to form the mineral tobermorite (C5S6Hs) at about 11 0 C. When the temperature is raised to 150 C, the tobermorite transforms into xonotlite 15 (C6SeH). These phases improve the stability and mechanical integrity of the cement at high temperatures; however, owing to slight differences in their respective densities, the recrystallizations can cause the permeability of the set cement to increase. At 250 C truscottite (C7S,2H3) begins to appear Owing to the manner in which truscottite crystallizes, a reduction in 20 permeability is usually observed. (Nelson, Well Cementing - "Thermal (:ements" 1990 9.1 - 9.19; J P Gallus et al, "Performance of Oil-Well Cementing Compositions in Geothermal Wells" SPE 7591 1978; L H Eilers et al, "High Temperature Cement Compositions - Pectolite, Scavvtite, Truscottite or Xontolite: Which Do You Want?" JPT 1983 1373-1377).

The degradation mechanisms described above are certainly not exhaustive, not least because the roles of other oxides present in the Portland cement, in particular alumina and ferrites, have been ignored, but they suffice to demonstrate the complexity of the phenomena occurring, without even considering the reaction products between the 30 cement matrix and brines. In nature, in a geological medium, cement is thus meta-

stable. In contact with brine, its mineralogical repertoire is modified by ion exchanges, which modify its physical qualities.

The problems of cement disintegration when the temperature is raised are more severe when developing very deep wells, or injecting steam to recover highly viscous oils, or in geothermal wells to exploit geothermal energy. Geothermal wells are drilled into formations that contain superheated brines. At surface the brine is converted to steam 5 which is then used to produce electricity. Steamflooding is a process where steam in injected into formations that contain viscous oil. The viscosity of the oil is reduced, allowing it to be pumped to surface. The temperature and pressure conditions encountered in these "thermal" wells vary between 200-300 C and 10 MPa - 95 MPa, respectively.

1() An approach explored in work by Roy et al. ("Potential New High Temperature Cements for Geothermal Wells" AIME International Symposium on Oilfield and

Geothermal Chemistry, Houston, TX 1979, pp 153-161) showed that the mineral anorthite was promising as a binder in geothermal well cements. However, the 15 practical problem of formulating a pumpable cement slurry with a predictable working time remained to be solved.

Other approaches to the problems of high temperature cements can be found in US 5,900.053; US 6,143,069; US 6,332, 921; IJS 6,367,556; WO 01/70646; and GB 20 0203729.9.

The present invention resides in the recognition that rocks of basic compositions (poor in silicon, rich in calcium) maintain low porosity and a good mechanical resistance during weathering and can be considered as good natural analogues to conventional 2S cements. Anorthite, grossular, prehnite and zeolite occur in basic rocks of magmatic origin, transformed in metamorphic or hydrothermal conditions.

The object of this invention is to develop novel formulations suitable for cementing oil, gas. geothermal wells or the like, based on a Portland and alumina type cement 30 tending to form mineral phases such that the properties of the cement matrix, in particular regarding compressive strength, do not degrade in practice under the effect oi a rise in temperature and pressure (250 C-300 C; 20 MPa) and/or chemical attack (brines).

The present invention provides a method of designing a cement slurry, comprising: determining a temperature to which the cement slurry will be exposed in situ; determining a stable, thermodynamic equilibrium composition of a CaO-A1203 S SiO2-H20 (CASH) mineral system at the determined temperature; determining proportions of cement and mineral oxides required to provide a mixture having the determined composition of the CASH mineral system; and defining a series of particulate materials of predetermined particle sizes and densities, and comprising the cement and mineral oxides in the determined proportions such that, when mixed with 10 water, a pumpable slurry is formed.

The proportions of cement and mineral oxides required to from a stable cement can be determined from general thermodynamic rules, phase relations between minerals and fluids and minerals, activity models and homogenous and heterogeneous phase I 5 equilibria known and developed for natural silicon- and calcium-rich rock systems of mafic origin to predict the mineralogy and chemical behaviour of Portland cements or derived compounds set in environments comparable to those encountered in geological settings such as hydrothermal fields of metamorphism (temperature greater i

than 1 50-200 C' pressure greater that liquid-vapour transition curve).

The method provides cement slurries that promote anorthite formation during the hydration of operating slurry by adding specific modifiers. Anorthite, characterized by solid solutions, exhibits the best adaptation to severe chemical environment. i 25 In order to promote the formation of anorthite, specific modifiers have to be added to the cement powder. At the same time, the final slurry density often has to be as low as possible (e.g. 1.4 to 1.5 sg) (due to the low fracture gradients generally experienced in geothermal wells) while maximizing the solid fraction of the slurry to decrease the permeability and increase the compressive strength.

To achieve this, the physics of optimizing the packing between the different components of the material has to be mastered. The particle size distribution of each I component has to be optimum. For example, the solid components can be provided in

( three distinct particle size fractions to optimize the amount of solids in the mixture.

This technique allows the use of large amounts of solids while making the slurry still easily pumpable.

5 Class G cement can conveniently be used as the hydraulic binder. Cenospheres or other hollow particulate material of very low density (0.7 to 0.8 sg) can be added to decrease the slurry density. Silica flour and aluminum modifiers are the other components of the blends.

10 Apart from the cement for which the particle size is typically defined by the manufacturer, particle size of cenospheres, silica and aluminum modifiers are carefully selected in order to achieve maximum packing of the blend. The size and the crystallinity of the different blend components allow control of the kinetics of anorthite formation.

The present invention will now be described by way of examples, with reference to the accompanying drawings, in which: Fig. I shows a plot of blend compositions in the CaO-AI2O3-SiO2 ternary diagram; Fig. 2 shows mineral phases known to be stable in natural calcium-rich systems 20 plotted of in the CaO-AI2O3-SiO2 ternary diagram (H2O in excess); Figs. 3a and b show SEM microphotography of set cement matrix obtained in a "xonotlite dominant" sample; Figs. 4a and b show SEM microphotography of set cement matrix obtained in an "anorthite dominant" sample; and 25 Figs. Sa and b show SEM microphotography of a set cement matrix of a further sample. Different cement blends are prepared according to the method of the invention. Table 1 summarizes the physical properties of the blends and slurries. All slurry properties 30 have been measured as per API specifications (API Recommended Practice for testing

Oil-Well Cements and cement Additives (1990) API Spec.10 Fifth Edition (07.01.1990)). The compositions of blends in the quaternary system (CaOSiO2-

Al2O3-112O) are shown in Figure 1.

s

Table 1: Properties of the slurries No slurry #3 #4 #5 tI2 # I Density (g. cm) 1.4 1.49 1.49 1.94 1.89 W/C ratio 1.3 0.58 0.81 1.07 0.69 Solids Volume 0 40 0.59 0.59 0.43 0 42 Fraction Free Water O O O O O Cement Cement Cement Cement Cement Blend Silica flour Silica flour Silica flour Silica flour Silica Flour Cenospheres cenospheres cenospheres Silica Aluminum Aluminum Aluminum modifier modifier modifier Plastic viscosity 33/6 257/29 109/9 53/21 149/11 (cP)/Y'eld value aRer mixing (Ibs/100 sq) Plastic viscosity 16/13 09/3 54i6 30/15 85/7 (cP)/Yield value after 20 man conditioning (Ibs/100 sqR) Reading on FANN 11 21 15 35 4 mod.35SA vscosimeter (Rl-Bl-Fl) Gel at 10 min _ Thickening Time 6 00 4:54 4:16 5 09 4:52 (hours) Transit Time 0:06 0. 02 0:05 0 15 0:05 Compressive Strength aRer 24 hours at 400 F 50 psi 3:43 4.40 4:34 6 02 500 psi 11 55 5:53 6:15 6:55 4:29 24 00 680 2100 1480 2420 6000

The slurries are mixed following the API procedure. Mixing is done in a Waring 5 Blender. All solid components are dry blended before being added to the water containing the liquid additives.

Once the slurry design has been optimized, samples are prepared for curing by pouring the mixed slurry into a Scm x 5cm x 5cm steel molds. Five cubes are 10 prepared for each formulation.

The molds containing the slurries are cured for three days at 300 C over 21 MPa in a standard oil well cement-curing chamber in water. The temperature is gradually increased to 300 C over a 6-8 hour heat up period, to minimize possible cracking of

the samples due to thermal expansion. The temperature is maintained at 300OC and 21 MPa for 28 days, after which the system is allowed to cool down to room temperature. 5 The cured set cement samples obtained after 28 days exposure at 300 C, are drilled in 2.5 x 5 cm cores. These set samples are exposed to the production brine at 300 C over 21 MPa in the high pressure - high temperature geothermal reactor for 28 days.

lle brine is regularly changed (every 3 days) to keep the solution saturated and to simulate as close as possible actual well conditions, where the brine is an "open 1 () system". The heating rate to reach 300 C is 6 C/min.

Samples are then removed, kept under water at room temperature before being tested for the following physical property measurements: 15 o Uniaxial compressive strength performed on 2.5 x 5 cm cores in using a compressive machine o Water Permeability Mineralogical examination is carried out by x-ray powder diffraction using CuKrx 20 radiation. Scanning Electron Microscope images are used to describe morphologies of reaction products and to complete the XRD analysis.

Two types of brines are used to simulate the chemical attack. Table 2 shows the chemical composition of a production brine circulated in a Japanese geothermal well 25 at 1300 meters with a production temperature around 300 C. The pH of this brine is 2.6. Table 2: Chemical composition of the production brine (Brinel) Ions mg/l Ma 3120

( Ca i 1460 Mg 270 Cl 9570 S04 201

SiO2 144 Fe 648 The table 3 shows the composition of a California brine.

5 l able 3:Chemical composition of the field brine (Brine 2)

ions NaCI 200 KCI 23

CaCI2 63 MgCI2.6H20 The results of the mineralogical composition of the different samples before and after 10 brine attack are presented in the following table.

I Formulation Number #3 #4 #5 #2 #1: Density g/cm3 1.49 1.49 1.49 1.94 1. 94 t3rine 1 or 2 attack at 300 C Before Aflcr Before After Berorc Before ARer Bctore Attcr: (Tuesdays brine 1 3nne brine Brine brine bane Bone brine 13nuc attack I attack I attack attack 2 attack 2 attack att ack attack _ attad: Xonotitte +++ +++ ++ ++ ++ ++ ++ +++ +++ C6S&H + + + +

Anorthite ++ ++ +++ ++++ + ++ +++ CaAl2Sl2og + + Anorthite,sodlum, intermediate ++ I (Ca.Na)(Si.AI)408 is

Alblte +++ NaAISi10 Warakitc ++ ++ ++ CaAl:Si,()', 2H.0 Prehnite (zeolithe) ++ + [SiAlO,]Ca'Al(OH), __ Epidote 1-2 ++ ++ [Sio7l[sio4](oH) Al2ocaAl' _ Garnet ++ [sio4]3Aloca _ Quartz + + +++ SiO2 _ _ Corundum + + ++ CAIRO, _

Prevailing phase; Ad+ phase in appreciable amount; phase in fairly good amount; + phase in small amount.

5 Figure 2 presents in the same quaternary system CASH as shown in Figure 2, the blend compositions from I able I and the different mineral phases of basic rock after hydrothermal alteration known as stable at 300 C.

Two areas can be defined: 10. Area 1, outside the two joins Epidotepectolite and Prehnite-pectolite Area 2 near or inside these two joins where the formation of xonotlite can be avoided or limited.

In area I, two compositions have been tested, # 1 and #3 with 1.94 and 1. 49 g/cm3 15 densities respectively. The phase responsible for the cementing properties within this region is mainly xonotlite.

With the low-density cement composition (#3), anorthite in a fairly good amount is easily detected. Its composition contains more alumina than the one at higher density.

( After brine 1 attack, the set cement matrix shows deleterious signs and the water permeability after one month in the Brine 1 increases. Fine needles of xonotlite shows dissolution signs particularly on the core borders.

5 In area 2, the compositions #4, #5, #2 are investigated. The composition #2 has a density of 1.94 g/cm3 and is cured in the brine 2. Compositions #4 and #5 have a lower density, 1.49 g/cm3. Phases responsible for the cementing properties within this area are mainly anorthite, whatever the slurry density. For the composition # 4, a high temperature calcium zeolite, wairakite well detected by XRD and SEM analysis 10 contributes with anorthite to favourable cementitious properties at 300 C. The composition #5, containing more alumina and less silica but having the same density, shows a mineralogical "assemblage" more complex: relic phases as silica and alumina are well detected and wairakite is less observed with SEM. This composition seems to be slower than #4.

If the slurry density is initially higher (#2), anorthite stays the main binder but prehnite, epidote are well detected. Relic phase as silica and alumina are still not completely consumed after one month at 300 C. Wairakite is not detected.

Composition #2 contains a part of silica coarser than the one added in the composition 20 #5. That can explain a lower reactivity after one month resulting from lower water saturation in silica useful to forte this phase.

After brine attack, whatever the slurry density, anorthite evolutes to a sodic plagioclase member, albite that shows well the good adaptation of anorthite to 25 chemical alteration.

The compressive strength and the water permeability of the set cement after curing without or with the brine are presented in table 5.

30 Table 5: Compressive strength before and after brine attack and water permeability after brine attack for the different compositions.

Formulation Number #3 #4 #5 #2 # I Density g/cm3 1.49 1.49 1.49 1.94 1.94 BHST F 300 300 300 300 300

Compressive Strength (psi) at BHST afterl month 900 2332 800 2207 6352 Water permeability (mD) 0.66 0.16 0.2 Compressive Strength (psi) at 830 2500 2300 5400 BlIST afterl month in brine Water permeability (mD) after 2.81 0.26 brine attack Even though the value of the compressive strength is lower for blend #2 than # I before brine attack, after brine attack, the formulation #2 shows no decrease in the compressive strength. If the brine is directly added as mixing water, this trend is still 5 observed contrary to the trend obtained with the formulation #1 where the compressive strength decrease of 55% of its initial value.

I'he formulation #4, at a density of 1.49 shows no variations in the compressive strength before and after brine attack and only a slight increase in permeability.

he composition #5 exhibits the lower compressive strength with a chemical system not completely reacted.

Electron microscopy gives morphological information that completes the chemical, 15 physical, mechanical and diffractometric data. The set cement matrices provided Prom formulation #3 and formulation #4 are observed and compared before and after chemical attack. Figures 3a and b, and 4a and b present the set cement matrix obtained with the composition #4. Anorthite is well detected before and after brine attack in the interstitial matrix and crystallizes as interlocking platelets, which can 20 contribute to a superior compressive strength measurement (Figures 3a and b). This phase is also met in some cavities observed in this low- density system. Again anorthite crystallizes under interlocking platelets, plugging the cavity (1. igure

( 4a).Wairakite is well observed in the interstitial matrix under cubic crystals and seems well stable after brine curing (Figure 4b).

Figures Sa and b present the set cement matrix obtained with the composition #3. The 5 samples have been cut in the core near the free surface and inside the core. In view of XRD analysis, xonotlite as fine needles, is the primordial hydrate. Figures 5a and b show the effect of brine curing through the core: in the core near the free surface, the signs of xonotlite dissolution in the interstitial matrix are more significant than inside the core. In view of the SEM observations, the increase of water permeability after 10 brine curing can be explained: 0. 66 mD and 2.8 mD before and after brine attack, respectively. In view of the mechanical and durability results, the composition #4 seems to be the best candidate for geothermal applications. The permeability and the compressive 15 strength are not affected after brine attack at 300 C. Indeed, the results have shown that the faster anorthite forms the lower strength retrogression is observed: at 300 C, the composition #4 exhibits the higher compressive strength with a complete reaction after one month at high temperature, the composition #5 exhibits the lower compressive strength with a chemical system not completely reacted.

The fact that the original material component (cement, cenosphere, aluminum modifiers) have completely reacted can partially explain this durability. Indeed, unreacted silica, and aluminum modifiers react with brine to give secondary phases which are not necessarily stable under these conditions.

Anorthite is the calcic member of the solid solution plagioclase that is well known to be stable at high temperature and which can adapt to brine circulation by ions exchanges in its network. Wairakite is a high temperature Ca-zeolite that is able, like all zeolite, to absorb ions in its structure that can be a good property with brine 30 circulation. Its stability domain is well known in the system Na20-CaO-MgO-A1203 SiO2-H20 in the range 0-500 C and Ol-lOkbar involving the phase's anorthite, grossular, prehnite and others. This phase does not exist in formulation #2 at higher density. A fraction of the silica in Composition #2 is coarser (silica) than the silica

contained in Composition #5 (silica flower). That can explain a lower reactivity after one month resulting from lower water saturation in silica useful to form this phase.

This also shows how the kinetics of the targeted phases (anorthite, wairakite) could be controlled by the particle sizes of the different components of the blend.

The present invention provides lightweight formulation using conventional Class G cement for temperatures up to 300 C. The set cement can show very good indications of durability (low permeability and no strength retrogression).

10 In the process used to design the slurry, the temperature stable phases (anorthite, wairakite) are selected; aluminum modifiers and silica oxides are added to the dry cement so that the elemental composition of the blend corresponds to the phases selected; the control of the particle size distribution and the relative amount of those minerals allow their addition at high concentration while leaving the slurry easily IS mixable and pumpable; and the kinetics of the targeted phases formation is then controlled by adjusting the crystallinity and the particle sizes of the different solids.

The methods of the invention can be used to design cement blends of a suitable cement and other solid additives which, when mixed with sufficient water can form 20 pumpable slurries. In well cementing, the slurries are pumped into the well in question and allowed to set to provide zonal isolation and well stabilization. The compositions can also be used to line pipes. In this case, a cement slurry is placed inside a pipe, which is then rotated, typically at high speed to coat the slurry onto the inside of the pipe where it is allowed to set.

Claims (11)

1 A method of designing a cement slurry, comprising: i) determining a temperature to which the cement slurry will be exposed 5 in situ, ii) determining a stable, thermodynamic equilibrium composition of a mineral system, analogous to the cement when set, at the determined temperature; iii) determining proportions of cement and mineral oxides required to 1 () provide a mixture having the determined composition of the mineral system; and iv) defining a series of particulate materials of predetermined particle sizes and densities' and comprising the cement and mineral oxides in the determined proportions such that, when mixed with water, a pumpable 15 slurry is formed.

2 A method as claimed in claim 1, wherein the mineral system is a CaOAI2O3 SiO2-H2O (CASH) mineral system.

20

3 A method as claimed in claim I or 2, wherein the cement is a Portland or alumina type cement.

4 A method as claimed in any of claims I - 3, wherein the series of particulate materials is defined so as to promote the formation of anorthite.

5 A method as claimed in any preceding claim, wherein the series of particulate materials is selected to provide a pumpable slurry of density less than 1.94.

6 A method as claimed in claim 5. wherein the step of defining the series of 30 particulate materials includes selecting a low density particulate component having a density of less than 0.8 sg as part of the series.

7 A method as claimed in any preceding claim, wherein the step of defining a series of particulate materials includes the step of defining a series of discrete particle size fractions to optimise the pacing of the solid particles in the slurry.

5

8 A cement blend comprising a series of particulate materials, including cement, defined in accordance with the method of any preceding claim.

9 A cement slurry, comprising a cement blend as claimed in claim 8 mixed with sufficient water to form a purnpable slurry.

10 A method of cementing a high temperature well, comprising: i) designing a cement slurry according to a method as claimed in any of claimed I such that the temperature of the well is used as the determined temperature; 15 ii) forming a slurry from the defined series of particulate materials and water; and iii) pumping the slurry into the well.

11 A method of lining a pipe for high temperature fluids, comprising: 20 i) preparing a cement slurry as claimed in claim 9, the determined temperature being the temperature of the fluids in the pipe; ii) placing the slurry in the pipe; and iii) rotating the pipe so as to deposit the slurry evenly on the inner surface of the walls of the pipe.