Well Test Requirements for Evaluation of Coalbed Methane Development Potential
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AbstractThis paper presents methods for designing and analyzing prefracture and postfracture injection/falloff tests and multirate prefracture and postfracture injection/falloff tests and multirate production tests in coalbed methane reservoirs. These tests should production tests in coalbed methane reservoirs. These tests should be conducted on selected wells in a coalbed methane pilot test. Pilot testing is the best way to evaluate the productive potential of coalbed methane reservoirs. A properly designed testing program can minimize the expense and maximize the reliability of estimating reservoir properties that are essential to a thorough evaluation of a coalbed methane reservoir.IntroductionA comprehensive testing program is necessary to evaluate the economic viability of large-scale coalbed methane field development. The testing program includes pre- and postfracture injection/falloff tests and a multirate production test. These tests provide a reservoir description for predicting long term productivity provide a reservoir description for predicting long term productivity and ultimate recovery from coalbed methane wells.Large-scale coalbed methane field development requires significant initial investment. Most coalbed methane reservoirs require hydraulic fracture stimulation, artificial lift, water disposal facilities, and complete well pattern development before significant gas production can occur. Coalbed methane wells normally produce their highest water rates early in their producing lives and before peak gas production rates occur. A testing program that can provide peak gas production rates occur. A testing program that can provide data for predicting productivity and ultimate recovery is essential to making prudent economic decisions.A five-spot pilot program should be drilled on 80 acre spacing so that the performance of an infill location can be approximated by the interior well. Well performance in coalbed methane reservoirs is strongly dependent on the amount of pressure interference between wells. Interference allows the reservoir pressure to be lowered rapidly. This allows gas to be released from the coal matrix and flow to the producing wells through the coal's natural fracture network or cleats. Ely, et al., describe the necessary operational steps to take in designing and carrying out a coalbed methane pilot.PREFRACTURE TESTING PREFRACTURE TESTING Prefracture injection/falloff tests should be designed to estimate permeability and initial reservoir pressure. An optimal well test permeability and initial reservoir pressure. An optimal well test design will result in accurate estimates of reservoir properties at the least expense. Test design requires estimated and assumed values for well and reservoir parameters. Table 1 lists the information required to design a prefracture injection/falloff test and sources of these data. The Gas Research Institute has released several reports on the various coalbed methane provinces in the U.S. over the past several years. These documents can be used to obtain estimates of many reservoir properties if no other source is available.Well test design includes estimating the test duration necessary to obtain reliable estimates of reservoir properties. Design also includes calculating rate and pressure limitations so that the proper equipment will be used to conduct the test. Duration of well bore storage distortion is normally the most critical factor in determining the proper test duration. Also, the fracture pressure of the tested interval must not be exceeded during injection operations. If a well is hydraulically fractured during the injection phase of the test, the data for the entire test will likely be meaningless. The pressure constraints also lead to low injection rate constraints. Special pumps are usually required for these tests but most service companies do have the necessary equipment.Table 2 lists the equations used in prefracture test design. Tables 3 and 4 provide data and calculations which illustrate the use of these equations. Table 3 lists typical data for a well located in the Black Warrior Basin. Table 4 shows the calculations made for the test design. The first column of calculations shows results for a wellbore that remains full of water throughout the test. The second column of calculations shows the results for a moving gas-liquid interface in the well.The first calculation estimates the wellbore storage coefficient. The changing liquid level case results in a wellbore storage coefficient that is 640 times greater than when the wellbore remains full of water.P. 521
