Measuring the Hydraulic Fracture-Induced Deformation of Reservoirs and Adjacent Rocks Employing a Deeply Buried Inclinometer Array: GRI/DOE Multi-Site Project
Authors: Branagan P.T. , Warpinski N.R. , Engler B. , Wilmer R. 
Published: December 2007

Abstract
A vertical inclinometer array consisting of six biaxial tiltmeters was cemented behind pipe at depths between 4,273 and 4,628 ft. This wide-aperture array provided real-time tilt profiles corresponding to a series of seven hydraulic fractures being conducted in a nearby offset well in a fluvial sandstone reservoir. Array profiles for three KCl water fracs indicated that height growth was confined to the gross thickness of the reservoir despite extensive fracture length extension. Long-term monitoring of the array suggests that a substantial residual frac width remained long after fracture closure occurred.
For two 400-bbl linear gel minifracs, some height growth was observed but it was not extensive. Tilt amplitudes related to expanded frac widths were found to increase as would be expected with these thicker frac fluids. When cross-linker and proppant were included in the last fracture, tilt-derived heights were seen to grow rapidly extending into the bounding layers as the more complex fluids entered the fracture system.
This inclinometer array was one of several independent, yet complimentary, fracture diagnostics tools that included cross-well multilevel microseismic arrays, FRACPRO and a remote fracture intersection well. Their purpose was to provide integrated field-scale data regarding hydraulic fracture dynamics and geometry that would be used to construct accurate fracture mapping and diagnostic techniques.

Introduction
A series of field-scale fracture experiments were conducted at the Gas Research Institute (GRI)/Department of Energy (DOE) Multi-Site (M-Site) with the explicit intent of gathering specific data regarding the time-dependent characterization, mapping and areal dimensions of hydraulic fractures. These experiments were part of a larger fracture diagnostics program involving three sands of the Mesaverde group designated - from deepest to shallowest - the A, B, and C Sands. Portions of the site selection process and A-Sand experimental results and conclusions have been previously reported, the B-Sand experiments have recently been concluded, and the C-Sand experiments are currently underway.
For the recently concluded B-Sand experiments, various treatments were performed including KCl water minifracs, gel minifracs and a propped hydraulic fracture. Researchers then relied on several independent diagnostic methods to characterize and define the induced hydraulic fracture(s). A discussion of the initial experiments, field operations, information regarding the full array of diagnostics along with results is contained in complementary papers. This paper provides the details, observations, and results from one of those diagnostic methods, a deeply buried vertical array of inclinometers, or tiltmeters. This inclinometer array was designed to measure the mechanical deformation of the formation in the vicinity of the fracture before, during and after each fracturing experiment.

Background
Surface tiltmeter arrays have been used for many years in assessing hydraulic fracture geometry. Tiltmeters operate like a level in that they can detect any deviation or tilt in position from horizontal that may be caused by the hydraulic fracturing process.
The arrays are normally deployed at surface locations in shallow sand-filled holes, e.g. 10-ft to 30-ft deep. Typical deployment occurs in an extended elliptical or circular geometries surrounding the expected surface projection of the fracture azimuth. The composite data from this horizontally deployed array is inverse modeled to provide the engineer with a most probable set of fracture parameters, such as dip and fracture azimuth, related to the interpretation of measured deformation.
The appeal of directly monitoring mechanical rock deformation was that it was one additional independent method of characterizing certain aspects of an induced hydraulic fracture.
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