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+ Is That Frac Job Really Breaking New Rock?

Mike Mullen, Halliburton, Milt Enderlin, Gearhart

With all that horse power sitting on location shaking the ground while pumping a frac job it is really hard to imagine that the fracturing treatment is doing down hole. Is it breaking into new rock in a linear elastic mode or is it just opening pre existing planes of weakness in the reservoir? Let's consider the implications of both scenarios.

If the rock is failing down pre existing planes of weakness in the rock, the frac job will tend to stay in the weakened zone without growing through weak shale barriers. This could explain why most of the frac models show poor containment in tight sands yet tracer logs show good frac containment. How about Microseismic surveys during frac treatments? They tend to see a rather large stimulated reservoir volume and shorter lengths that is modeled. If the fracture treatment is breaking new rock the created fracture will tend to be more like what is modeled in current frac simulation models.

So how do you tell if you are opening a pre-existing weakness in the rock or breaking new rock? A novel concept to do this is to integrate the geomechanically determined stress state of the rock with hydraulic fracture diagnostics. By comparing the estimates of minimum horizontal stress from both disciplines one can determine which type of fracturing one is doing. This leads to an improved post stimulation diagnostic analysis and trouble job analysis.

+ The Stress Polygon: Still the Tool for Evaluating and Integrating the Scale of Stress Inlfluence.

Structure, as in structural geology, can be considered the response (fold, fracture, fault, and/or compact) of rocks to stress. Since structures can extend over a range of scales, from millimeters to kilometers, then it follows that extent of stresses influence has a similar range of scale. In 1987 (Zoback and Mastin) and again in 1990 (Moos and Zoback), the Stress Polygon method for visualizing the relationship of the magnitudes of overburden stress, maximum horizontal stress, and minimum horizontal stress was introduced and used. When used as a map overlay, as printed on clear plastic, the Stress Polygon provides valuable insight into the stress magnitudes and stress direction that were in place when the larger scale structures, depicted on the map, were created. But perhaps more importantly, integrating the Stress Polygon with smaller scale borehole information, such as breakouts and drilling induced tensile cracks, the Stress Polygon becomes an extent of stress influence scaling tool. This presentation will demonstrate the map overlay technique and how integrating the Stress Polygon with borehole information can be used to evaluate the scale of influence of a particular stress state.
Summary / Discussion The procedure / concepts just presented focuses on the relative orientation, with respect to north, of the stresses at two locations to infer the extent of stress influence. But what about the stress magnitudes? The evaluation of the Sh magnitude at the active fault region and vertical well location can be independently constrained by careful evaluation of Sv and constructing a Stress Polygon for both locations, assessing the coefficient of friction (u) of the preexisting plane or weakness (fault in this case), evaluating the faulting style, studying drilling / completion history, appraising presence or absence of breakouts and /or tensile cracks, and knowing the strength of the rock and the Pore pressure [see Zoback (2008) for clarification]. If Sh magnitude at both locations appears to be about the same and the stress directions are the same, then there is a chance that the same stress conditions extend from the fault region to the well location. The caveat, of course, would be the possible range in the SH magnitude, but even then SH is somewhat restrained by the Stress Polygon for the faulting style present and the strength of the rocks (end-cap constraints).

If you would like to acquire a genuine ESIST for yourself so that you too can generate static electricity by sliding it around on maps and other things, please send your name and address to marketing@gearhartcompanies.com and we will send you one at no charge.

+ Up-Scaling of Rock Mechanical and petrophysical Properties for Grain Scale to Log Scale by Point-Load and Wedge Indentation Tests.
Ramos, G. G. and Chin, L.Y.
ConocoPhillips, Bartlesville, OK, USA
Enderlin, M.B.
Gearhart Co., Fort Worth, Tx, USA
Copyright 2008, ARMA, American Rock Mechanics Association

This paper was prepared for presentation at San Francisco 2008, the 42nd US Rock Mechanics Symposium and 2nd U.S.-Canada Rock Mechanics Symposium, held in San Francisco, June 29-July 2, 2008.
This paper was selected for presentation by an ARMA Technical Program Committee following review of information contained in an abstract submitted earlier by the author(s). Contents of the paper, as presented, have not been reviewed by ARMA and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.
ABSTRACT: This is a laboratory and field technique of scaling-up rock mechanical and petrophysical properties, such as compressive strength and bulk compressibility, from grain-scale to log-resolution scale. The main scaling tools are the point-load wedge penetrometers. The objective is to extrapolate the physical and mechanical properties from core samples to larger volumes of numerical geomechanical models grids or cells. The scaling method starts at grain-scale using a conventional point-load indenter that creates a conical depression onto the rock surface. A single PLP indentation has a scale of investigation of a few grain diameters, but contiguous point-indentations, or linear wedge indentations expand the investigated volume geometrically. The geometry of the dent is a function of rock strength, modulus, petrophysical properties, indenter geometry and applied normal force. The depths and widths of point- and line-indentations were calibrated with plug-measured uniaxial compressive strength, P-wave velocity, and modulus. Laboratory and field cases are presented. Grain-scale strengths show wide variances relative to whole-core strengths. However, the penetrometer indices of compressive strengths are shown to compare favorably with conventional measurements on plugs, core ultrasonic velocity, scratch-tester, and with those predicted by the wells sonic and density logs. With additional log data on porosity and clay-volume, scaling up to a simulators grid scale followed usual geomodeling techniques.