The cyclic injection process to improve water flooding in carbonate reservoir was evaluated in laboratory experiments, as well as analytical and numerical simulations. Laboratory cyclic injection experiments were performed on a carbonate rock sample with an artificial horizontal fissure. Live oil was established by in-situ recombination of dead oil with hydrocarbon gas at reservoir temperature and bubble point pressure. The experiments were designed by numerical simulation of the cyclic process. Analytical modelling was done to evaluate cyclic injection sensitivity to critical reservoir and process parameters. The cyclic water injection experiments above oil bubble point pressure increased oil recovery by additional 2.9% of Oil Originally In Place (OOIP) above ordinary water flood. This effect can be attributed to the hasten imbibition of water into the matrix during pressurisation half-cycles and capillary retained water in the fine pores in the matrix resulting in oil counter current flow from matrix to the fracture during de-pressurisation half-cycles. Cyclic injection below bubble point pressure, designed to ensure gas saturation not exceeding the critical level, yielded additional recovery of 5.9% of OOIP. This effect may be credited to the energy of released gas, expelling the matrix oil into the fracture. The experimental results indicate significant potential of cyclic injection to improve micro level displacement efficiency of waterflood in carbonate reservoirs. If cyclic water injection is applied at field scale, sweep efficiency improvement from flow pattern redistribution could make an additional contribution. This has been reported by several field cases in the US, China and Former Soviet Union (FSU). Introduction The concept of cyclic injection is based on (1) pulsed injection and (2) alternating waterflood patterns [1–4]. Cyclic injection appears to have the greatest potential in heterogeneous, permeability-contrast reservoirs, with light, high compressibility fluids. Cyclic or pressure pulsing injection has been successfully applied in a number of sandstone and fractured carbonate oil fields in Russia, USA and China [1, 5–8]. Reduction of water production and acceleration of oil extraction rates were observed at several field applications. In the cyclic process, water injection rates are changed between high and low values, in a periodic fashion. The cycle periods at field scale are typically in the range of days to months, much different from the known pulsed pressure technique, where several pressure pulses are applied in time intervals of minutes. Laboratory and field applications of cyclic water injection indicate that additional oil recoveries in the range of 2–15% can be achieved with significant reduction in water cut levels, making the process very attractive and profitable [13,14,15].
Step Rate Test (SRT) is commonly used to estimate formation parting (or fracture opening) pressure for stimulated wells. SRTs may also focus on changes of well performance at different rates / pressures that is of special interest for stress-sensitive reservoirs such as fractured carbonates. Installation of permanent downhole gauges (PDG) and running SRTs on injection wells at the Ekofisk field gave a chance to improve the understanding of reservoir and well performance. Analysis of these SRTs also resulted in further development of SRT interpretation techniques. An approach to SRT interpretation, combining analytical pressure-rate (p-Q) curve analysis and step-by-step Pressure Transient Analysis (PTA) with numerical simulation of SRT, was suggested and tested. Applying this approach to Ekofisk SRTs has shown that the p-Q analysis may be used for diagnostics of well performance changes, while step-by-step PTA enables decomposing of well (skin factor) and reservoir (conductivity) effects with estimation of corresponding well-reservoir parameters as pressure or rate functions. Numerical simulation confirmed that pressure dependent conductivity estimated from the step-by-step PTA is the governing factor in matching SRT history. Special attention was paid to the uniqueness of SRT interpretation using suggested approach. Reaching infinite acting radial flow (IARF) regime at each step of a test provided unique parameter estimates as shown by example of a stimulated slanted injector. Being too far from IARF at end of each step will make the interpretations more uncertain. Different sets of changing parameters estimated from SRT interpretation could provide satisfactory match in numerical runs as was illustrated by example of a horizontal injector with multiple induced fractures. Comparison of interpretation results for different wells integrating additional field data is a possible way to reduce this uncertainty. Finally, some hints to designing, conducting and interpreting SRTs of different types of wells in fractured carbonate fields are given, using Ekofisk field experience.
Cyclic waterflooding is an IOR-method that improves oil production in heterogeneous reservoirs with high-permeability contrast. The concept of the method is based on pulsed injection and alternating of waterflood patterns. The main effect induced by the cycling of wells is oil production increase accompanied by water production decrease. The production increase is achieved by improved sweep efficiency in low permeable zones of a reservoir non swept by traditional waterflooding process. The cyclic water injection process was successfully applied in a number of sandstone and carbonate oil fields in Russia, USA and China. The important advantages of the method are virtually zero additional cost and simple implementation procedure. The uncertainty with the method is related to understanding the IOR mechanism, ability to accurately model the process and design a field application. The paper presents the results of the study of cyclic water injection and oil production at a North Sea heterogeneous sandstone reservoir. The study consists of the field history analysis, pre-screening cyclic efficiency estimation and numerical reservoir simulation to design the field application of the IOR-method. The field history analysis shows the presence of cycles in injection and production and their influence on water-cut change. Pre-screening analytical tool was used to perform a wide-range sensitivity analysis with respect to rock-fluid parameters, heterogeneity, cycle length and pressure conditions in order to understand the mechanism and to estimate the IOR potential. Finally the sector model reservoir simulation was used to optimize the cyclic process by alternating the waterflood patterns and to design the field application. The simulations show decrease of water production and improvement of oil production by up to 3% with short time and by 5% with long time cycles. Introduction The cyclic waterflooding improves sweep efficiency in heterogeneous reservoirs. In the IOR-method a combination of two processes of (1) pulsed injection (production) and (2) alternating of waterflood patterns is used. The cyclic waterflooding was successfully applied in a number of sandstone and carbonate oil fields in USA1,2 and Former Soviet Union 3,4,5. The cyclic injection potential was evaluated in a number of studies 3,4,5,6,7,8. Analytical estimation and numerical simulations have shown significant potential with oil production increase by up to 10% and reduction of water-cut by up to 20%. Recent experimental studies 9 have demonstrated improvement of oil recovery under pulsed pressure conditions at laboratory scale. The uncertainty of cyclic waterflooding is related to understanding of the IOR mechanism, ability to model and predict efficiency of the process, and to design a field application for specific reservoir conditions. In this paper we analyze production and injection data, discuss and evaluate parameters that affect cyclic waterflooding, estimate efficiency of the method for different scenarios of field application. Cycling effects which occurred in the production history of a heterogeneous sandstone reservoir were analyzed to evaluate their influence on water-cut behavior. The analytical tool 8,10 was used for screening of the cyclic injection. A wide range sensitivity analysis was performed to estimate efficiency of the cyclic process. Based on the analytical screening results the numerical simulations of cyclic waterflooding were carried out on a sector reservoir model. IOR potential of cyclic reallocation of water injection volumes between well patterns was also estimated.
Pressure transient analysis (PTA) is traditionally used to characterize well and reservoir parameters from well tests based on shut-in periods. PTA was widely used for reservoir management and decision making before reservoir simulation became the main tool. Presently more and more reservoirs are surveyed by permanent (downhole) gauges. These gauges provide vast amount of pressure transient and rate data which may be interpreted using improved PTA approaches to gain more knowledge on reservoir dynamics. This has opened new prospects for PTA applications in field studies. Permanent pressure and rate measurements cover both well flowing and shut-in periods occurring during normal operations. These measurements allow for analysis of time-lapse pressure transients and comparative interpretation of flowing and shut-in periods. The analysis of time-lapse data provides time-dependent description of well-reservoir parameters. The comparative interpretation gives understanding of flowing reservoir properties which are often different from those estimated from well shut-in periods as in the classical PTA. These approaches provided basis for an improved methodology of interpreting permanent pressure measurements, where the scope of the standard PTA application may be extended to integrate new data sources. Application of the developed methodology has been demonstrated with data from fields on the Norwegian Continental Shelf. For many wells the comparative interpretation revealed significant difference in well-reservoir parameters estimated from flowing and shut-in periods. It was also found that the flow regime near a well may vary. For example, the dominance of a hydraulic fracture observed during a shut-in period may be significantly reduced during the flowing period with simultaneous changes in reservoir conductivity. Dynamic behavior of natural and induced fractures due to pressure (stress) changes, wellbore cross-flows and variable contribution of reservoir layers are considered to be possible reasons for these effects. Analysis of time-lapse pressure transients provided description of long-term changes in well-reservoir parameters, e.g. reservoir conductivity, hydraulic fracture properties and boundary effects. The key advantage of applying this methodology is characterization of the more representative flowing, rather than shut-in, well-reservoir parameters as well as their permanent monitoring during reservoir lifetime. The monitoring may reveal ongoing changes in the reservoir characteristics, production impairment risks etc. The methodology uses data readily available for many fields and provides an updated description of well behavior and hydraulic reservoir properties. The reward may be improvement in everyday field operations and well performance optimization, reduced uncertainty of input to reservoir models and better decision making through enhanced model predictability.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.