Slug Testing in Multiple Coal Seams Intersected by a Single, Vertical Fracture

stimulated wells completed in commingled reservoirs, i.e., This paper examines tic transient pressure response from slug reservoirs in which the layers communicate only at the wellbore tests in wells completed in multiple, hy&aulically fractured coal and not in the fractures or reservoir. Under these conditions, our seams. We developed a model for stimulated wells producing model is applicableeither for wells in which the layers have ken from commingled reservoirs, I.e., reservoirs in which the layers stimulated with separate fracture treatments but are produced communicate only at the wellbore and not in the fracture or through a common wellbore, or for wells completed in multiple layers intersected by a single fracture in which negligible gravity reservoir, Whh this model, we have investigate.dthe effects of various reservoir properties on the slug test pressure behavior and effects and low verticalfracturepermeabilitieslimitverticalflow. +avc found that the characteristicpressure responsedepends more on the contrast between layer properties than their absolutevalues. Wc rdso pr~s~ni a method for correlating the slug test pressures DEFINITIONS AND MODEL DEVELOPMENT from multi-layer and single-layer reservoirs, thus allowing us to use singic-htyertype curves to analyze slug tests, Following the work of Spath, et u1,5 and Johnston and Lee! we model a coinmingled reservoir having multiple, hydraulically fractured layers, The model considers a reservoir in which each INTRODUCTION layer can have distinct rcswvoir and fracture properties; however, each layer is an isotropic, homogeneous, infinite, horizontal Conventional slug test type curves and analysis techniqucslta arc medium with a uniform thickness, permeability, and porosity, In based on single.layer, radiabflow models. Since coal seams arc addition, each layer is assumed to contain a slightly compressible rclativel~ thin compared to conventional natural gas reservoirs, fluid with viscosity and compressibility that are mdcpendcnt of economically . !ablc coalbed methane wells often must bc pressure, Finally, wc assume the initial pressures in all layers arc complctcd in multiple scams, In addition, the typical range of equal and single-phase flow exists at all times. Under these pcnneabilitkx observed in coal requires that wells be hydraulically conditions, the Laplace transform of the total, dimer,sionlcss fractured, especittll~ for the critical dcwatcring phase early in a pr:ssure response for the !aycrcdsystems,~is WCI1’S productive hfc. Thcrcforc, {hcse convcntiomd slug test analysis techniques arc not valid for either wtluating wells complctcd in multiple coal scams or assessing the post-fracture p’(u) = -----l-— i &j hj 1 potentialof hydraulicallyfracturedcoalbed methanewells, ,,, ,,, ,,, ,, .,,,. ,!!,!.,,,,,,,,,. .,. !(1) j., Ill P~j(”) Kawtki, et al,s examined the pressure behtwiorduring slug tests in WCIISintersecting infinite-conductivity, vertical frncxres, where u is tt Laphtcc spi~ccvariable, n is the numbs’ of layers, Similarly, Rushing, et td! investigated slug tcstin in wcus with andj is the layer index, ! finite.conductivity, vertical fractures, Both stu ies, however, were limited to single-layercases. Karsaki, et al,s dso studied the The dimensionless pressure, ~’j(u), in thejlh layer is computed shq test pressure response in unstimuhttedIaycredreservoirs, but with Cinco-Lcy,et aL’s7 model which considers a WC1l intcrscmd their study only consldcrcdhornogwteous,two-layer forrrmtions, by a fully-penetrating, tinitc.conductivity, vertical fracture, l’hc fracture is rnodcledas a homo ~eneous,uniform slab, Becausethe The purpose of this paper is to investigate the transicm pressure fructurc width is assumed to‘be much smaller than the fracxurc bchiwior during slug tests in wells completed in multiple, length and height, the model assuxs flow in the fracture is linear hydraulically fractured COJIscams, Wc dcvclopcd a model for and fluid influx at the fracture ti s is negligible, In addition, the model assumesfluid production 10 m the reservoir to the wellbore =-s and illustratio).sat cad of pa= occurs only through the fracture, Further, since the fracture