ABSTRACT:

Hydraulic fracture height is one of the most difficult parameters to measure yet understanding height growth is becoming increasingly salient in the economic success of unconventional wells in multi-layer structures, particularly for projects with increased well density development. Downhole tiltmeter fracture mapping by passive monitoring of elasto-static microdeformation offers high sensitivity to fracture height. This work aims at presenting a workflow to integrate advanced microseismic analysis and tiltmeter fracture mapping to resolve dimensions of fracture with a non-uniform opening. The algorithms are implemented in a real-time fracture monitoring program which selects the best fit and superposes final-state and transient models on measured micro-deformation. We apply the presented technique to synthetic and field case studies and, for the first time, present transient tilt characteristics using heatmap visualization of slow deformation (tilt waterfall). Our motivation for the present study is to take advantage of the newly developed downhole instruments that convey a combined array of geophones and tiltmeters and can be installed at greater depth and temperature to monitor and evaluate fracture to as hot as 177°C (>12000ft).

1. Introduction

Hydraulic fracturing in unconventional reservoirs is a complex process controlled by the pumping parameters, rock mechanical properties, in-situ stress state, and multi-scale discontinuities (e.g., layering and interfaces, faults, natural fractures). Thereby it is poorly characterized by standard models unless discrepancies are resolved by introducing fudge factors (Warpinski et al. 1994). Downhole Tiltmeter Fracture Mapping (DTFM) is a unique technique that measures induced microdeformation near the fracture face and unravels the evolution of the volumetric distribution of fluid-driven fracture during treatment as well as after pumping stops. Fracture height, dip, volume, azimuth, opening, horizontal components, and complexity are among the parameters that impact the tilt response and can be potentially determined by DTFM if enough tiltmeters are located optimally. As depicted in Fig.1, the field deployment of DTFM to monitor the underground operation entails placement of at least one vertical, linear and wireline-conveyed array of tiltmeters in an offset well (Wright et al. 1998b). The array of tiltmeters is conveyed to the same depth range targeted by the treatment well. It is carefully deployed to ensure that enough data can be recorded from above and below the fracture depth. The acquisition unit samples tilt sequentially in time at each tiltmeter normally with a sampling rate of < 5 HZ.

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