A case study involving power water injection in the fractured Arab-D carbonate reservoir in a Saudi Arabian field is discussed. The study was conducted to investigate the role of injection operations in the initiation and propagation of induced fractures and their communication with nearby faults, and to provide a methodology for early detection of the induced fracturing process.
The study involved analysis of data gathered from step-rate, falloff, flowmeter tests, as well as injection rate and pressure data over the history of the injection operation, followed by well test modeling and hydraulic fracture modeling (HFM).
Most of the eight wells studied showed the existence of fractures, corresponding to a rise in injection pressure beyond the fracturing gradient or formation parting pressure (FPP). Skin and injectivity indices obtained from the falloff tests were found to be good indicators of fracturing behavior, based on which most of the studied wells were inferred to communicate with the natural fracture system or super-permeability streaks. HFM showed that induced fractures could reach a half-length of up to 1400 ft, to various heights depending upon the injection rate and permeability. The distance to the nearest fault obtained by superimposition on a 3-D seismic interpretation was found to vary from 500 to 2,000 ft. At high injection rates, fractures were found to grow out of reservoir into underlying tight formation, which could lead to loss of injected water. For controlled fracture height, which may lead to more efficient injection operations, preparation of injection rate guidelines was recommended.
Introduction
Water injection is commonly practiced in depleting reservoirs for pressure support, to increase total recovery, etc. High injection pressures may sometimes exceed the formation parting pressure (FPP), creating/opening fractures that may communicate with the natural fracture system in a naturally fractured reservoir. The horizontal and vertical extent of the induced fractures, and their interaction with the natural fracture system, determines the efficiency of the injected water for the desired purpose.
The present study deals with power water injection in an oil-producing, fractured carbonate reservoir in Saudi Arabia. Eight power water injection wells were identified for the study. The purpose of the study was to:investigate if the injection operations lead to the initiation and propagation of fractures,to determine the extent of the induced fractures which could establish a communication with nearby faults, and,to determine a methodology for early detection of the induced fracturing process.
Withdrawal of fluids from hydrocarbon reservoirs results in a decrease in pore pressure which in turn leads to an increase in effective stress on rock matrix. Such a situation may lead to the occurrence of pore collapse in reservoirs having weakly cemented, porous rocks. It is considered to be a potential problem in several producing reservoirs. Numerical simulation of a compacting reservoir due to pore collapse requires an appropriate constitutive model. Consequently, a constitutive model based on the concept of elasto-plasticity using isotropic hardening is developed to predict pre and post-pore collapse behavior of reservoir rocks. An experimental study is carried out on a high-porosity rock susceptible to pore collapse for different stress paths. The developed constitutive model is tested with respect to two different materials exhibiting such behavior. Parameters for the model are evaluated based on the experimental results, highlighting the procedure involved. Further, the data is used to demonstrate the strengths and the weaknesses of the constitutive model. Experimental data for the second material is obtained from literature. Satisfactory agreement is achieved between experimental data and model predictions.
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