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Abstract

"Impulse fracture" is an injection test used to determine formation permeability and reservoir pressure. The test consists of a small-volume water injection to create a short fracture and a shut-in period afterwards to record pressure falloff. The pressure falloff after fracture closure is used to deduce permeability and reservoir pressure. The fracture can pass the near-wellbore damaged area and have the true formation exposed to flow transients. Also, fracturing may sometimes be difficult to avoid for injections in low-permeability formations.

The theory and analysis of impulse fracture are based on an instantaneous- source solution to the diffusivity equation. Numerical simulation examples and field case studies are used to support the validity of the analysis. The applicability of impulse fracture in gas reservoirs is demonstrated by numerical simulation results from a multi-fluid-bank simulator and by field cases. The impulse-fracture injection test is an economical and simple means to determine formation permeability. The test can be conducted in conjunction with mini-fracture or micro-fracture stress-test injections.

Introduction

"Impulse fracture" is an injection test for determination of formation permeability and reservoir pressure. It consists of an injection and a shut-in period. During the injection, a small volume of water is injected into the well, and a short fracture is created in the formation. During the shut-in period, the pressure falls off and the fracture closes. The pressure is recorded before and after the fracture closure. The late-time pressure-falloff data, after the fracture closure, is used to deduce permeability and reservoir pressure.

The impulse-fracture injection test is similar to the slug test or the impulse test. In a slug test, a small volume of fluid is injected into, or withdrawn from, the reservoir to create a pressure disturbance in the formation. The subsequent pressure response is analyzed to estimate the reservoir flow properties. As a simple and economical means of evaluating reservoir properties, the slug test has many applications in the petroleum industry.

Like the slug test, the theory and analysis of the impulse-fracture test are based on an instantaneous-source solution to the diffusivity equation. When the duration of the injection period is short compared with the shut- in time, the injection can be considered as an instantaneous source. The flow regime of the test can be identified based on the characteristics of the instantaneous-source solution, and the reservoir parameters can be deduced.

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