Hydraulic Fracturing: Modeling Hydraulic-Fracture Treatments in San Juan Basin Coals: Fully Functional 3D Fracture Model
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 107972, "Lessons Learned From Modeling Hydraulic- Fracture Treatments in Coals Using a Fully Functional 3D Fracture Model in a San Juan Basin Project," by Muthukumarappan Ramurthy, SPE, Halliburton, and Bill Lyons, SPE, Anadarko Petroleum Corporation, prepared for the 2007 SPE Rocky Mountain Oil and Gas Technology Symposium, Denver, 16–18 April. The paper has not been peer reviewed.

The two main objectives for a hydraulic-fracture model in coals are optimization of job design and placement, and post-treatment diagnostics. Fully functional 3D models can be used to obtain good estimates of the fracture dimensions. A grid-oriented fully functional 3D fracture simulator with shear decoupling can be especially useful in coals for post-treatment diagnostics if sufficient input data can be fixed from logs and diagnostic fracture-injection tests (DFITs).

Introduction

The full-length paper contains an extensive literature review on hydraulic fracturing and modeling in coal. Successful placement of hydraulic-fracture treatments was an issue in the upper Fruitland coals of this project area. The Fruitland coals in this area were divided into basal (S8 coal) and upper (S9 coal) units. Typically, S8-coal fracture treatments were relatively easy to place while the S9 coals were more difficult to treat across the region, resulting in occasional near-wellbore screenouts. West of Farmington, almost all S9-coal units were more difficult to treat, resulting in screenouts. The completion program in this project area was suspended to investigate, analyze, and address the problem accordingly. Four problem-well stimulations across the project area were history matched with a grid-oriented fully functional 3D planar-fracture simulator. The analysis revealed that most perforations were not open before the treatment (causing excessive perforation friction) and that there was severe pressure-dependent leakoff (PDL) with an estimated PDL coefficient as high as 0.011 psi−1 in the S9 coals. Significant job-design changes needed to address these issues. The full-length paper details lessons learned by use of this fracture model in understanding and then solving the problems faced during stimulation in this project area.

Reservoir Information and Geology

The West Farmington project, shown in Fig. 1, is in the northwest part of the San Juan basin, spread across six townships. The coal rank in this area ranges between high-volatile-C and high-volatile-B bituminous coals, and all are slightly undersaturated. The target coal seams are the lower Fruitland (S8) and upper Fruitland (S9) coals. The target seams increase in depth from west to east in the project area. The S8 coals are continuous in nature and vary from one to two seams, while the S9 coals are very discontinuous and vary from one to three seams. The average net thickness of the S8 coals is approximately 15 ft, while the combined average of all the S9 coal seams is 16 ft.