Prefracturing Pressure Transient Testing: East Texas Cotton Valley Tight Gas Play
Authors: Bostic J.N. 
Published: December 2007

Performance analysis of low permeability gas wells requires an accurate knowledge of formation flow capacity (kh) and initial static reservoir pressure (pi). These parameters are used in calculating original gas-in-place, predicting well rates and reserves, and assessing economics. In addition, they are key design and evaluation work. Formation flow capacity and initial reservoir pressure are best determined from prefracturing pressure transient tests; however, conventional testing techniques have proven inadequate in low-permeability formations.

Amoco's experience in prefracturing pressure transient testing of the 0.001 to 0.01 md (0.001 to 0.01 × 10(-3) m) gas sands in the Cotton Valley Formation of East Texas has led to the establishment of the following three-step procedure to enhance accurate prefracturing pressure transient data collection:Preflow test for static pressure determination(pi).Pressure drawdown test to determine producing characteristicsPressure buildup test to determine formation flow capacity (kh).

Severe wellbore storage effects, which are characteristic of low-permeability formations, have made it necessary to utilize some method of downhole shutin in Cotton Valley to obtain the required pressure data within a reasonable time. Though methods exist to analyze pressure transient data with storage effects, it has been found that a more accurate analysis is possible when the storage effects have been minimized. possible when the storage effects have been minimized. Increased costs resulting from the use of downhole shutin devices may be partially offset by carefully planning the test procedure. planning the test procedure. Actual field results along with predicted results from a 1-D radial, single phase gas reservoir simulator illustrate the effectiveness of the outlined technique and the need for accurate and reliable data.

Introduction

Low permeability gas wells must be stimulated with MHF treatments to be commercial. Since it is difficult to determine reservoir parameters after fracturing, reliable "prefracturing" pressure transient data is essential to the evaluation of a low permeability gas reservoir. This testing is needed to determine formation flow capacity (kh) and initial static reservoir pressure (pi) which are required for proper MHF design as well as rate, reserve and gas-in-place determination.

One of the low permeability gas plays that Amoco is currently developing is the Cotton Valley Sands of East Texas. Amoco's experience in prefracturing pressure transient testing of these sands has been pressure transient testing of these sands has been generally successful when proper test design has been combined with the necessary mechanical technology.

This study presents certain aspects of Amoco's prefracturing pressure transient testing experience prefracturing pressure transient testing experience in the Cotton Valley. It covers the objectives and reasons for testing, test design, mechanical aspects of testing, and data analysis. Computer simulated and actual field examples are included for illustration. An additional purpose of this study is to present a systematic thought process necessary for present a systematic thought process necessary for adequate test design data analysis in other similar plays.

NEED FOR PREFRACTURING TESTING

The reasons for running prefracturing pressure transient tests in low permeability gas plays are similar to the reasons for running them in any gas reservoir. They include a need for accurate measurements or estimates of initial reservoir pressure, formation flow capacity, and relative condition of the near wellbore area (skin zone). The primary differences between a pressure transient test in a low permeability gas well and a more conventional pressure transient test are the difficulties in pressure transient test are the difficulties in obtaining reliable data and its analysis.

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