RU2473804C1 - Method of hydrodynamic investigations of injection wells - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: method involves pumping cycle of working fluid with constant flow rate to injection well and further shutdown of the well with recording of pressure decline curve (PDC). In addition, repeated pumping cycle is performed with recording of pressure stabilisation curve (PSC); at that, pumping flow rate is chosen so that the cycle pressure is higher than formation fracturing pressure. Then, as per the investigation results and pressure decline curve and pressure stabilisation curve method there determined are formation conductivities h_PDC and kh_PSC, and then, ratio of those conductivities: kh_PDC/kh_PSC. Based on the criterion of kh_PDC/kh_PSC<1 ratio, the conclusion is made on crossflow availability, and the conclusion is made on its intensity as per 1- kh_PDC/kh_PSC.

EFFECT: improving informativity of hydrodynamic investigations of injection wells at diagnostics of unstable cracks and evaluation of their effect on the formation development.

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Description

The invention relates to oil production, and in particular to technologies for conducting, interpreting and analyzing the results of field geophysical (PIP) and hydrodynamic (well test) studies in injection wells.

It has been established that with high productivity of wells, unstable fractures of hydraulic fracturing are formed. The mechanism of their formation is similar to hydraulic fractures, which are created artificially to increase the productivity of production wells. Unstable cracks appear naturally due to the high injection rate, when the pressure at the bottom exceeds the rock's strength limits (fracture pressure). Unstable cracks are characterized, firstly, by the fact that they exist only during injection and disappear when the well stops, and, secondly, by the fact that their sizes vary depending on the intensity of injection.

In injection wells, the main purpose of which is to maintain reservoir pressure, the formation of unstable cracks, although it is an unplanned result, but, as a rule, is not critical for the operation of the well and the formation.

The exception is cases where the presence of a crack contributes to the connection of additional thicknesses of the reservoir to the well. The wings of the crack, spreading horizontally and vertically, reveal closely located permeable layers. As a result of reservoir connectivity along a dynamic fracture, injection fluid is observed to leave not only in the design interval, but also in additionally uncovered thicknesses.

This negatively affects the development, as it reduces the effectiveness of the reservoir pressure maintenance system. The negative effect of unstable cracks also lies in the fact that the effectiveness of measures to level the injectivity profile is reduced.

Existing research methods cannot always accurately diagnose the presence of a crack, and most importantly, they cannot determine whether a crack is an overflow channel. This does not allow to make an informed decision on optimizing the injection mode for localization of distribution.

For the diagnosis of reservoir connectivity (cross-flow), unsteady thermal studies are currently being carried out. The technology of work on the well during such studies (see, for example, RF patent No. 2289689, 12.24.2004) consists in conducting a cycle of pumping water into the well with a temperature different from the natural temperature of the rocks, then shutting down the well, recording a series of thermograms and evaluating thermograms rate of restoration of the initial temperature. The flow is judged by the abnormal rate of temperature recovery.

However, this technology has the following significant disadvantages:

1) it is difficult to take into account the effect of changes in depth in the radial direction of the thermal properties of rocks and fillers in the wellbore;

2) it is impossible to distinguish whether overflow occurs in an unpressurized annular space or an unstable crack.

A more effective technical solution, also widely used in practice, is a method for diagnosing reservoir connectivity based on the results of well testing performed by cycles: operation of an injection well with a constant flow rate, subsequent shutdown of a well and registration of a pressure drop curve (efficiency) (see, for example, section 13.7.5 monographs Ipatov AI, Kremenetsky MI “Geophysical and hydrodynamic control of hydrocarbon field development”, p. 582-585).

In this case, the fact that the main cause of the overflow is an unstable fracture that existed before the well stopped, is indirectly judged by the abnormally low negative value of the skin factor, estimated by the results of well testing.

The disadvantage of this method is that in itself the formation of a crack does not always lead to overflow. Overflow occurs only if the crack drains non-perforated reservoirs in the host rocks. And it is impossible to identify the fact of drainage in a single test cycle using the pressure recovery curve (HPC).

The objective of the invention is to increase the informativeness of hydrodynamic studies in the diagnosis of flows over unstable cracks and assess their impact on the development of the reservoir.

The solution of this problem is achieved by the fact that after a cycle of injection into the injection well of a working fluid with a constant flow rate and subsequent stop of the well with the registration of the pressure drop curve (COP), a repeated injection cycle is carried out with the registration of the pressure stabilization curve (KSD). Moreover, the injection flow rate for the repeated cycle is chosen so that the pressure in it is higher than the fracture pressure.

Further, according to the results of hydrodynamic studies using the efficiency and CSD methods, the formation conductivity for these cycles is determined, and then the ratio of these conductivities is α = kh _efficiency / kh _CSD .

Conductivity in the efficiency cycle (kh _efficiency ) corresponds to the reservoir power drained by perforation. KSD cycle conductivity (kh _KSD ) is the sum of the conductivities of the perforated collector power and the power that communicates with perforation only along the crack.

In the absence of overflow, the conductivities determined by the efficiency and KSD practically do not differ from each other, and their ratio is close to unity α≈1.

In the event of an overflow along an unstable crack, the parameter α is always less than unity. Moreover, its value is the lower, the greater the power of non-perforated collectors drained by a crack.

Thus, by the anomalously low value of the conductivity ratio, one can judge about the presence of overflow, and by the value of the conductivity ratio - about the drained power (and, therefore, indirectly - about the intensity of the overflow).

This makes it possible to unambiguously diagnose the hydrodynamic connection of the strata by an unstable crack and estimate the intensity of the flow over the crack.

An example of a practical implementation of the proposed method is presented in figures 1 and 2.

Figure 1 shows a graph of hydrodynamic studies at one of the injection wells ******* field.

Figure 2 shows a comparison of the results of hydrodynamic studies for efficiency and CSD cycles in the log-log scale, where:

1 - pressure curves;

2 - the corresponding curves of the logarithmic derivative;

s _KSD , s _Efficiency - the average distance between the pressure curve and the logarithmic derivative for the radial flow regime, characterizing the magnitude of the skin factor in the cycles of KSD and efficiency;

kh _KSD , kh _KVD - the position of the radial asymptote to the curve of the logarithmic derivative, characterizing the value of the conductivity of the formation in the cycles of KSD and efficiency.

The injection well before hydrodynamic studies worked with an average flow rate of 200 m ³ per day for 250 hours. After that, the well was stopped, and a pressure drop curve was recorded in it (cycle with the index of "efficiency" in figure 1). Based on the results of the interpretation of the efficiency (curves with the index of "efficiency" in figure 2), the formation conductivity (kh _efficiency = 26.6 mD · m) and the skin factor (s _efficiency = -4.7) were estimated.

A negative value of the skin factor indicates that an unstable fracture was formed in the reservoir before the well stopped in the formation.

It is impossible to conclude whether a given fracture is an interstratal overflow channel for a given cycle.

Further, according to the proposed method, a repeated injection cycle was carried out with the pressure stabilization curve recorded (cycle with the “KSD” index in FIG. 1), and the injection rate was selected so that the pressure was higher than the fracture pressure.

The interpretation of this curve showed that in the injection cycle, the formation conductivity increased sharply (kh _KSD = 58.2 mD · m), which indicates the connection of additional non-perforated formations to the injection.

The conductivity ratio in the efficiency and KSD cycles was:

α = kh _Efficiency / kh _KSD = 26.6 / 58.2 = 0.46.

This value of the conductivity ratio (substantially less than 1) indicates a high intensity of the overflow.

Taking into account the fact that, according to indirect data, the permeability of the collector, opened by perforation, and nearby collectors are close, we can confidently conclude that as a result of the overflow, additional collector capacities connected to the injection, comparable in size to the perforated object.

Claims (1)

The method of hydrodynamic research of injection wells, including a cycle of injection into the injection well of a working fluid with a constant flow rate and subsequent shutdown of the well with the registration of the pressure drop curve (COP), characterized in that it additionally produces a repeated injection cycle with the registration of the pressure stabilization curve (KSD), moreover the flow rate during injection is chosen so that the pressure in this cycle is higher than the pressure of the fracturing, then, according to the results of studies on the efficiency and KSD methods, lyayut formation conductivity: kh _efficiency and kh _CDS, then the ratio of these conductivities kh _efficiency / kh _SDC, in the case of relations kh _efficiency / kh _CDS <1 is judged that there flow and largest 1-kh _efficiency / kh _RACs on its intensity.

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