Abstract

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:

  1. investigate if the injection operations lead to the initiation and propagation of fractures,

  2. to determine the extent of the induced fractures which could establish a communication with nearby faults, and,

  3. to determine a methodology for early detection of the induced fracturing process.

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