EVIDENCE OF PRESSURE DEPENDENT PERMEABILITY IN LONG-TERM SHALE GAS PRODUCTION AND PRESSURE TRANSIENT RESPONSES A Thesis by FABIAN ELIAS VERA ROSALES Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Approved by: Chair of Committee, Christine Ehlig-Economides Committee Members, Robert Wattenbarger Maria Barrufet Head of Department, Dan Hill December 2012 Major Subject: Petroleum Engineering Copyright 2012 Fabian Elias Vera Rosales ABSTRACT The current state of shale gas reservoir dynamics demands understanding long- term production, and existing models that address important parameters like fracture half-length, permeability, and stimulated shale volume assume constant permeability. Petroleum geologists suggest that observed steep declining rates may involve pressure- dependent permeability (PDP). This study accounts for PDP in three potential shale media: the shale matrix, the existing natural fractures, and the created hydraulic fractures. Sensitivity studies comparing expected long-term rate and pressure production behavior with and without PDP show that these two are distinct when presented as a sequence of coupled build-up rate-normalized pressure (BU-RNP) and its logarithmic derivative, making PDP a recognizable trend. Pressure and rate field data demonstrate evidence of PDP only in Horn River and Haynesville but not in Fayetteville shale. While the presence of PDP did not seem to impact the long term recovery forecast, it is possible to determine whether the observed behavior relates to change in hydraulic fracture conductivity or to change in fracture network permeability. As well, it provides insight on whether apparent fracture networks relate to an existing natural fracture network in the shale or to a fracture network induced during hydraulic fracturing. ii DEDICATION To the lord, our God. To my current and future family, on earth and heaven. To my city, Guayaquil, and my homeland, Ecuador To my father and to my mother. iii ACKNOWLEDGEMENTS I would like to thank my committee chair, Dr. Ehlig-Economides, and my committee members, Dr. Wattenbarger and Dr. Barrufet, for their guidance and support throughout the course of this work. Thanks also go to my friends and colleagues, the department faculty, staff and the research team by Ibraheem, Han, Bo, Sippakorn, Oyewande, Jenny, Jessica, Dongmei, Tianyu, Kyung Jae, Kaushik and Jacqueline for making my time at Texas A&M University a great experience that will last forever. Finally, thanks to my mother and father for their spiritual support beyond borders and faith to each other which during five years consequently gifted them their first son, author of this thesis. iv NOMENCLATURE A = Area, acre b = Fracture width, ft cf = Initial rock compressibility, 1/psi cg = Initial gas compressibility, 1/psi ct = Initial total compressibility, 1/psi cw = Initial water compressibility, 1/psi CfD = Dimensionless hydraulic fracture conductivity h = Formation thickness, ft k = Formation permeability, md kfw = Fracture conductivity, md-ft Lw = Horizontal well length, ft m(p) = Real gas pseudopressure, psi2/cp nF = Number of hydraulic fractures pi = Initial pressure at formation depth, psi pwD = Dimensionless bottomhole flowing pressure pwf = Bottomhole flowing pressure, psi Q = Cumulative production, scf q = Gas production rate, Mscf/d rw = wellbore radius sg = Gas saturation, fraction T = Time, hr T = Formation temperature, °F v tDxF = Dimensionless time respect to fracture half-length te = Material balance time, hr te, elf = End of linear flow material balance time, hr telf = End of linear flow actual time, hr 3 V = Volume, ft 3 Vp = Pore Volume, ft xF = Hydraulic fracture half-length, ft xs = Hydraulic fractures spacing, ft zw = Vertical distance to lower boundary, ft Greek Variables ϕ = Porosity, fraction μ = Gas viscosity, cp 휕p = Derivate with respect to pressure = Diffusivity = Exponential decay factor Subscripts D = Dimensionless e = Material balance time elf = End of linear flow i = Initial p = Constant pressure R = Constant rate vi TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION ................................................................................................................. iii ACKNOWLEDGEMENTS .............................................................................................. iv NOMENCLATURE ........................................................................................................... v TABLE OF CONTENTS .................................................................................................vii LIST OF FIGURES ........................................................................................................... ix LIST OF TABLES ...........................................................................................................xii CHAPTER I INTRODUCTION ....................................................................................... 1 1.1 Problem Descriptions ............................................................................................... 1 1.2 Objectives ................................................................................................................. 2 1.3 Methodology ............................................................................................................ 2 1.4 Organization of this Thesis ...................................................................................... 3 CHAPTER II LITERATURE REVIEW ............................................................................ 5 2.1 Matrix-PDP .............................................................................................................. 5 2.2 Hydraulic Fracture-PDP ........................................................................................... 8 2.3 Natural Fracture-PDP ............................................................................................. 10 2.4 Detecting the Presence of PDP in Transient Pressure and Production Data .......... 14 2.5 Objectives ............................................................................................................... 18 2.6 Methodology .......................................................................................................... 18 2.7 Organization of this Thesis .................................................................................... 18 CHAPTER III SENSITIVITY STUDIES ON PRESSURE-DEPENDENT- PERMEABILITY ............................................................................................................. 20 3.1 Long Term Production Sensitivity Analysis .......................................................... 21 3.1.1 Matrix-PDP Cases ........................................................................................... 22 3.1.1.1 M-PDP Finite Conductive Fracture Case ................................................. 22 3.1.1.2 M-PDP Infinite Conductive Fracture Case .............................................. 25 3.1.2 Hydraulic Fracture-PDP Sensitivity Cases ...................................................... 28 vii 3.1.2.1 HF-PDP with Infinite Conductive Fractures ............................................ 28 3.1.2.2 HF-PDP with Finite Conductive Fractures .............................................. 31 3.2 Pressure Transient Sensitivity Analysis ................................................................. 34 3.3 Reduced versus Exponential Declining Permeability ............................................ 34 3.4 Chapter Summary ................................................................................................... 36 CHAPTER IV FIELD CASES OF PRESSURE DEPENDENT PERMEABILITY ....... 38 4.1 Fayetteville Shale Case .......................................................................................... 38 4.2 Haynesville Shale Case .......................................................................................... 45 4.3 Horn River Shale Case ........................................................................................... 59 4.4 Chapter Summary ................................................................................................... 68 CHAPTER V CONCLUSIONS AND RECOMMENDATIONS ................................... 70 REFERENCES ................................................................................................................. 72 APPENDIX A .................................................................................................................. 76 APPENDIX B .................................................................................................................. 82 viii LIST OF FIGURES Page Figure 2.1: Horner plots at different drawdown times. ...................................................... 6 Figure 2.2: Effective external stress acting on a reservoir. ................................................ 7 Figure 2.3: Propped fracture conductivity as a function of stress ...................................... 8 Figure 2.4: Effect of conductivity on long-term production .............................................. 9 Figure 2.5: Reduction of initial gas production rate because of PDP presence ............... 10 Figure 2.6: Importance of natural fracture system for optimal stimulation ..................... 11 Figure 2.7: Log-log plot showing decrease of the natural fracture