Variability in Coalbed-Methane Well Performance: A Case Study
Authors: Zuber M.D. , Reeves S.R. , Jones A.H. , Schraufnagel R.A. 
Published: December 2007

This paper presents a case study of three producing coalbed-methane wellswith various productivities in the Oak Grove field. productivities in the OakGrove field. Each well is examined in terms of geologic setting, stimulationeffectiveness, simulator-predicted recoveries, and observed well performance togain insights into the mechanisms responsible for variability in coalbedmethanewell performance. Local variations in geology and reservoir propertiessignificantly affected the properties significantly affected the effectiveness of hydraulic fracture treatments and, hence, gas production rates from thesewells. Treating production rates from these wells. Treating pressures werematched with a pressures were matched with a fracture model, and fracturegeometry was estimated. A reservoir model confirmed mechanisms responsible forproduction variations. The main mechanism affecting well performance variationsin the study wells performance variations in the study wells appears to be thein-situ stress setting, which was substantially elevated within fault blocks. High stress settings result in lower coal permeabilities and abnormally highnet treating permeabilities and abnormally high net treating pressures duringfracturing, which pressures during fracturing, which result in less effectivestimulations. Low coal permeabilities and short fracture lengths bothcontributed to the poor well performance observed.

Introduction

The Oak Grove field in Alabama is divided into four areas: the U.S. Steel Mining Co.'s development area, where vertical wells were drilled on closespacing with the primary goal of degassing the coal before mining; the Gas Research Inst. (GRI) research area, where evaluation of coalbed-methanereservoir properties and multiple completions research are under way; the Taurus Exploration Inc. development area, where 30 single-zone completion wellswere drilled in 1986 with the major focus on economic production of methane;and the area of production of methane; and the area of Taurus Exploration's1987 drilling operations (Fig. 1).

In 1986, Taurus Exploration initiated a 30-well drilling program in the Oak Grove field to maximize economically recoverable gas from the Mary Lee/Blue Creek coal group. A major goal during the first year of operation was to definethe most effective stimulation, dewatering, and gas production techniques. Fourstimulation production techniques. Four stimulation designs with various fluidtypes were tried, and initial production was used to evaluate the economicsuccess of each stimulation. Core samples and permeability data were obtainedfrom selected wells, and a lineament study was performed to aid in welllocation. Lineaments are surface expressions of subsurface fractures orfracture systems associated with various degrees of secondary permeability. Ref. 2 gives results of these permeability. Ref. 2 gives results of theseinitial wells.

As a result of these studies, reservoir permeability was determined to hehighly permeability was determined to he highly variable across the field, ranging from 20 to 30 md at the 1986 Taurus wells to 2 to 5 md measured at the GRI research site. Increased gas production was observed when wells were sitednear lineaments. The lowest-cost stimulation, using water as the treating fluidand small sand concentrations, was found to be at least as effectiveeconomically for these higher-permeability areas as other, higher-cost designs. On the basis of production from the original 30 wells, high-volume productionfrom the original 30 wells, high-volume water stimulation (250,000 gal [946m3]) of equal or larger volumes was chosen as the treatment of choice for the1987 Taurus development program. Average production from the 30 wells drilledduring the 1986 development program was 160 Mcf/D [4531 m3/d] with a highdegree of variability reported in Dec. 1986 to range from 50 to 230 Mcf/D [1416to 6513 m3/d].

This paper presents a case study of three producing coalbed-methane wellswith producing coalbed-methane wells with various productivities completed inthe Oak Grove field in 1987 with comparable stimulation treatments. Each wellis examined in terms of geologic setting, predictions of stimulationeffectiveness, simulator-predicted recoveries, and observed well performance togain insights into the performance to gain insights into the mechanismsresponsible for variability in coalbed-methane well performance. The threeanalysis wells, Taurus Wells 441, 442, and 456, were drilled and completed andbegan production in late 1987. The productivity of these three wells rangesfrom poor to productivity of these three wells ranges from poor to good inrelation to the historic well productivity of Oak Grove field wells. Detailedproductivity of Oak Grove field wells. Detailed geologic analysis and a wealth of reservoir data provide the basis for a description of geologic setting andcharacterization of reservoir quality for the three wells. Bottomhole pressure(BHP) data recorded during stimulation were used to predict hydraulic fracturesize and effectiveness for each well. Finally, the production history of eachwell was matched with a two-phase coalbedmethane simulator to confirm reservoircharacteristics responsible for variability of well production.

Field Operation

Background.

Because of the large number of single-zone-completioncoalbed-methane wells drilled by Taurus Exploration, standard procedures weredeveloped for siting, drilling, completing, fracturing, and initiatingproduction from these gas wells. In about 30% of the wells, slug tests are usedto measure permeability. From this information and selected coreholes, contourmaps of permeability and gas content are drawn to aid well placement. Inselected wells, BHP is measured to analyze the effectiveness of the stimulationtreatments.

JPT

April 1991

P. 468