Effect of Cemented Natural Fractures on Hydraulic-Fracture Propagation
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 160197, ’Examining the Effect of Cemented Natural Fractures on Hydraulic-Fracture Propagation in Hydrostone-Block Experiments,’ by B. Bahorich, J.E. Olson, SPE, and J. Holder, The University of Texas at Austin, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 8-10 October. The paper has not been peer reviewed.

Microseismic data and coring studies suggest that hydraulic fractures interact heavily with natural fractures, creating complex fracture networks in naturally fractured reservoirs such as the Barnett shale, Eagle Ford shale, and Marcellus shale. Because direct observations of subsurface hydraulic-fracture geometries are incomplete or nonexistent, properly scaled experimental research and computer modeling based on realistic assumptions are used to help understand fracture-intersection geometries. Observations of cores from the Barnett and other shale plays suggest that most natural fractures are cemented. The most significant finding of this research was that fracture-intersection geometries are complex. Results showed that bypass, separation of weakly bonded interfaces, diversion, and mixed-mode propagation are likely in hydraulic-fracture intersections with cemented natural fractures.

Introduction

The resulting geometry from hydraulic-fracturing treatments in rocks having natural-fracture weaknesses, typically pumped from multiple injection zones spaced along lengthy horizontal wells, is not clear. The best direct evidence of fracture geometry is from mine-back studies in which realistic fracturing fluids were injected into realistic wellbores. But those studies often were at very shallow depths or in igneous or metamorphic rock. These types of studies report hydraulic-fracture diversion along natural planes of weakness, such as natural fractures and bedding planes. Multiple fractures can propagate simultaneously and parallel to one another, but, eventually, under many circumstances, a single fracture plane will dominate. Coring across hydraulic-fracture trends has shown that such multiple parallel fractures exist in tight gas sandstones.

Laboratory experiments also investigate the physics of hydraulic-fracture complexity, with the advantage of better boundary-condition control and near-complete observation of fracture geometry. The drawback of laboratory experiments is potential issues with proper scaling to ensure that results are applicable to field operations.

The objective of this work was to examine the nature of hydraulic-fracturing interaction with naturally cemented or sealed pre-existing fractures. This investigation was motivated by observations in formations similar to the Barnett shale in the Fort Worth basin in Texas, where most natural fractures are observed to be filled with calcite or quartz. Inclusions (glass slides, Berea sandstone slides, and gypsum-plaster slides) were embedded in gypsum-plaster blocks to represent cemented natural fractures. Linear and crosslinked gel was injected into the blocks under a true triaxial-stress state in which the hydraulic-fracture path was designed to intersect the embedded inclusions (natural fractures). Results showed a variety of interactions between hydraulic and natural fractures that could improve understanding of the subsurface process and provide rea-son to develop more-accurate hydraulic-fracturing models.