Fracture Characteristics and Reservoir Behavior of Stress- Sensitive Fracture Systems in Flat-Lying Lenticular Formations
Authors: Lorenz J.C. , Warpinski N.R. , Branagan P.T. , Settler A.R. 
Published: December 2007

A model is presented that suggests that regional fracture systems commonly control permeability in flat-tying reservoirs. Such fractures are distributed in a continuum of sizes and occur in subparallel, en echelon patterns. Few high-angle, "orthogonal" fractures exist because this system patterns. Few high-angle, "orthogonal" fractures exist because this system is created by high pore pressures and relatively low differential horizontal (tectonic) stresses rather than by significant structural deformation. Interfracture communication occurs primarily at infrequent, low-angle intersections of fractures. Vertical continuity of such fractures through a reservoir commonly is limited by the numerous lithologic discontinuities inherent in nonmarine sandstones. This type of fracture system has been documented in Mesaverde rocks in the Rulison field of the Piceance Creek basin, northwestern Colorado, by studies of 4,300 ft [1310 m] of core from the U.S. DOE's three Multiwell Experiment (MWX) wells and by studies of the excellent nearby outcrops. Well test results and geologic data from core and outcrop support the model. The described natural fracture system has a significant effect on production and stimulation.

Introduction

Efforts to stimulate production of natural gas from tight, lenticular reservoirs of the western U.S. [including massive hydraulic fracturing (MHF) and nuclear stimulations] have not been notably successful because of the complexity of these reservoirs. While speculation on the cause of poor well performance has focused on such phenomena as fracturing out of zone, poorly log-defined reservoir properties (particularly with respect to gas in place), or high water saturations, few attempts have been made to examine the results in relation to specific flow mechanisms, especially natural fractures.

As this paper describes, the results of experiments conducted at the U.S. DOE's MWX site have shown that narrow, stress-sensitive. natural fracture systems have a significant effect on well testing, stimulation, and production activities. Yet the presence of these fractures might not be apparent unless detailed core analysis and well tests are performed. Although natural fractures are important production mechanisms in low-permeability reservoirs, the natural production mechanisms in low-permeability reservoirs, the natural fracture systems through most of the Mesaverde at the MWX site are not well interconnected. Rather, they are subparallel and poorly connected vertically and laterally. This paper describes the unconventional natural fracture system that exists in the Mesaverde for mation at the MWX site and proposes that similar systems are common in other flat-lying, lenticular reservoirs. It also describes the supporting core, outcrop, and well-test data and discusses the effects of such fracture systems on completion and production activities. The model is subject to refinement, but we are confident that. in most respects. it corresponds to the subsurface reservoir characteristics and conditions at the MWX site.

These results have important implications for reservoirs that have not been structurally deformed and for lenticular reservoirs in particular. The technology for stimulating these reservoirs will be particular. The technology for stimulating these reservoirs will be ineffective without an understanding of the interaction of stimulation treatments with natural fracture systems.

The MWX

The MWX project consists of three wells that penetrate and test the Late Cretaceous Mesaverde formation in the east-central part of the Piceance Creek basin. northwestern Colorado. The wells are arranged in a triangle with legs of 140 to 180 ft 143 to 55 m] at the surface, varying to 110 to 215 ft [34 to 66 m] at depth. Some 4,300 ft [1310 m] of core was taken from the Mesaverde formation in these three wells, a third of it oriented. The project is a field laboratory, combining geology, engineering, geophysics, and a number of analytical techniques to characterize low-permeability, lenticular, natural-gas reservoirs. The excellent data on reservoir characteristics are provided by abundant core. Complete log suites, close well spacing, and an extensive well-testing program. plus detailed studies of nearby outcrops of the target formation, have been combined to provide a well-controlled model of subsurface reservoir fracture networks.