Pressure Falloff Tests of New Coal Wells
- J.P. Seidle (Amoco Production Co.) | G.M. Kutas (Amoco Chemical Co.) | L.D. Krase (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- Low Permeability Reservoirs Symposium, 15-17 April, Denver, Colorado
- Publication Date
- Document Type
- Conference Paper
- 1991. Society of Petroleum Engineers
- 6.5.2 Water use, produced water discharge and disposal, 2.5.1 Fracture design and containment, 3 Production and Well Operations, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.6.3 Pressure Transient Testing, 5.6.4 Drillstem/Well Testing, 5.8.3 Coal Seam Gas, 5.3.2 Multiphase Flow, 5.4.2 Gas Injection Methods
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This paper demonstrates field application of pressure falloff testing and analysis methods to new coal pressure falloff testing and analysis methods to new coal degas wells in the San Juan Basin. These tests have provided absolute coalbed permeability, wellbore skin, provided absolute coalbed permeability, wellbore skin, and initial reservoir pressure. Effects due to free gas and gas desorption on pressure falloff tests are considered. The effects of sorption compressibility are shown to have a significant impact on skin but not on permeability. Influence of the duration of the injection and shutin periods on the design and analysis of coal well PFOTs periods on the design and analysis of coal well PFOTs is also discussed. Field examples of both pre- and post-stimulation falloff tests are presented. post-stimulation falloff tests are presented. Introduction
Over the past decade, technology for pressure transient testing of coals wells has developed rapidly. Much of this work has concentrated on developing techniques for testing coal wells after gas has been desorbed. A single interference test in coal wells has been reported. However, many operators need to evaluate the commercial viability of new coal wells and few investigators have developed pressure transient tests suitable for such wells.
Use of slug tests in coalbed methane wells was discussed by Koenig and Schraufnagel and examples of these tests in Warrior Basin coal wells were presented. Slug tests were found to be simple, reliable, and economic tests for characterizing new coal wells. However, slug tests have two disadvantages. The practical radius of investigation of these tests is roughly 100 wellbore radii, which is typically about 30 feet into the coal seam, and in slug tests the accuracy of the permeability is no better than the accuracy of the wellbore storage coefficient. Slug tests offer no independent method of wellbore storage determination, and errors in this coefficient lead to errors in the permeability.
The first effort to consider two-phase flow in coalbeds was done by Kamal and Six. A new pseudopressure was defined which incorporates both gas-water relative permeabilities and sorption isotherm properties. By permeabilities and sorption isotherm properties. By assuming gas instantaneously desorbs from the coal matrix, Kamal and Six developed a method which yields absolute coalbed permeability and wellbore skin. They applied the method to two San Juan Basin coal wells and reasonable results were obtained. This method requires relative permeability data and the sorption isotherm, both of permeability data and the sorption isotherm, both of which are often not available for coal wells, especially wildcats.
Design and interpretation of injection tests in the Black Creek coals of the Warrior Basin of Alabama were reported by Zuber, et al. Their reported tests appear to be dominated by wellbore storage, indicating further consideration needs to be given to test design. Zuber, et al., used a simulator for test design. Credibility of their results would be increased if the simulator had also been used to match actual test response.
The purpose of the work reported here was to develop practical methods for the design and analysis of pressure falloff tests in new coal wells. Existing pressure falloff tests in new coal wells. Existing technology for injection well testing was readily adapted to new coal wells. Many times the analysis of these tests is quite straightforward.
New coal wells are frequently completed in water saturated coalbeds with no free gas in the coal fractures (cleats). In such cases, all gas is held in the coal matrix by sorption and is released only when pressure in the cleats has been reduced due to water production. With no free gas present, water injection into a water saturated coal exactly satisfies the unit-mobility ratio assumption of classical pressure falloff tests (PFOTs) and those methods can be readily adapted to testing new coal wells.
In conventional falloff tests, injection time is often much longer than shutin time. In PFOTs of new coal wells, however, injection time needs to be as short as possible to minimize testing costs.
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