Comparison of Hydraulic-Fracture Orientation Techniques

Lacy, L. L.

doi:10.2118/13225-pa

Cited by 32 publications

(15 citation statements)

References 13 publications

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“…Similar activation of natural fractures by induced hydraulic fracturing has been observed in other geophone surveys. 26 From our analysis of the data recorded by Sandia, it thus appears that geophone tests by themselves cannot be used as indicators of stress orientation at Belridge. Some of the geophone data, however, may represent induced hydraulic fractures aligned with sH' For example, the unimodal azimuth of N13°E recorded in Well L is virtually the same as the NI5°E azimuth inferred for sH from borehole ellipticity within the same well and depth zone (see Table 3).…”

Section: Stress Directionsmentioning

confidence: 92%

Earth Stress Measurements in the South Belridge Oil Field, Kern County, California

Hansen

Purcell

1989

SPE Formation Evaluation

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Summary Earth stress magnitudes in the South Belridge oil field, determined from integrated density logs and microhydraulic fracturing tests, indicate that the vertical stress is generally the intermediate principal stress, except possibly at the deepest zone tested (2,100 ft [640 m]), where it is approximately equal to the lesser compressive horizontal stress. Azimuth of the greater horizontal stress and of induced hydraulic fractures, as measured or inferred by several different techniques, is N15 degrees E 15 degrees. Introduction Wells completed in the diatomite/porcelanite reservoir of the North and South Belridge oil fields (Fig. 1) usually are hydraulically fractured at initial completion. Current expectations are that primary recovery will be relatively low and that supplemental recovery techniques may provide a significant portion of the ultimate production from this major oil accumulation. The success of any supplemental recovery technique will depend on the geometry of the extensive hydraulic fracture system induced during primary production, as well as that of the natural fractures present. Because fracture geometry is determined by earth stresses acting at the time of fracture formation, we attempted to determine as precisely as possible the stress state in the diatomite/porcelanite reservoir at Belridge. The three principal earth stresses may reasonably be assumed to be oriented vertically and horizontally in this reservoir. A complete description of the stress state therefore requires determination of the magnitudes of the three stresses and the azimuth of one of the two horizontal stresses. The various methods used for determining stress state at Belridge are based on geological observation and borehole mechanics. Geological methods allow orientation and possibly relative magnitudes of the three principal stresses to he inferred. For example, in areas of recent or active faulting, the following stress regimes are believed to prevail. For high-angle normal faulting, SV greater than SH greater than SH, with the greater horizontal stress, SH, oriented along the direction of surface strike. For near-vertical strike/slip faulting, SH greater than SV greater than SH, with SH Oriented at about 30 to 45 degrees with respect to the fault. In the case of low-angle thrust faulting, SH greater than SH greater than SV, with SH perpendicular to the surface strike of the fault. Borehole- mechanics methods examine stress state at the borehole wall under conditions of tensile failure (hydraulic fracturing) or compressive shear failure (borehole sloughing). When a fracture is induced at a borehole wall and then extended into the formation, it tends to align itself in a plane perpendicular to the least principal earth stress. If, as is assumed here, the principal earth stresses are oriented vertically and horizontally, and one of the two horizontal principal stresses is the least of the three, then a hydraulically induced fracture will leave a vertical wellbore in a vertical plane and will maintain that orientation as it propagates outward into the formation. The induced fracture will align with the direction of SH. Thus, a measure of fracture azimuth gives the direction Of SH and vice versa. On the other hand, if the vertical stress is the least of the three principal stresses, then the fracture may still leave the well in a vertical orientation, but will tend to turn over and to become horizontal as it enters a region in which the earth's stresses are essentially undisturbed by the presence of the borehole. By the time a fracture has been extended 5 to 10 borehole diameters into the formation, it should be responding to earth stresses, which, at this distance from the borehole, are distorted by less than 1 %. 3 The closure pressure of the fracture should then very nearly equal the least principal earth stress, either SV or SH-The instantaneous shut-in pressure (ISIP) recorded following fracture initiation and propagation is sometimes a reasonably good measure of the fracture closure stress and in this Belridge study is considered to be a direct measure of the least earth stress. If there is sufficient imbalance between the two horizontal stresses, compressive shear failure of the borehole wan may occur, causing sloughing and hole enlargement along a preferred cross-sectional axis. In a vertical hole, enlargement generally will be greater along an axis aligned with the direction Of SH (see the Appendix). If there are known instances of out-of-round boreholes that can reasonably be attributed to stress-related shear failure, then oriented four-ann caliper surveys or borehole televiewer logs can be used to determine azimuths of the horizontal stresses. Stress Magnitudes During Oct. and Nov. 1980, 11 small-volume hydraulic fractures ("microfracs") were induced to determine the magnitudes of the principal horizontal stresses. The fractures were formed between straddle packers set in open hole at two or three different elevations in each of four closely spaced wells. Location and Procedure. The four wells selected for this study penetrated the contact between the Belridge diatomite and the overlying Tulare sandstone near the crest of the structure (Fig. 2). Three intervals were fractured in Wells A, B, and C and two intervals in Well D. Correlation of electric logs ensured that corresponding intervals were fractured in all four wells. The 8-ft [2.4-m] intervals between straddle packers in which the small fractures were induced fell within larger intervals that were later hydraulically fractured through perforated casing during routine completion operations for the four wells. Preparatory to microfracturing in open hole, the straddle-packer assembly was run in on drillpipe with the hole and pipe filled with the "standard" mud used fieldwide for drilling the diatomite. Mud density was nominally 8.9 lbm/gal [1066 kg/m3]. After the packers were set, fresh water was pumped in on top of the mud at a constant rate of 3 gal/min [11 L/min]. Pressure was recorded as a function of time both at the surface and in the interval between the packers by a surface-recording Amerada pressure gauge. A digital memory readout (DMR) pressure recorder was hung below the bottom packer to detect any pressure communication that might occur during the fracturing operation. Breakdown of the formation was followed by a succession of five or more periods during which mud was pumped into the induced fracture for 1 to 5 minutes, after which the system was shut in for 2 to 10 minutes. Fig. 3 shows the three pressure-vs.-time records obtained in Well B. The continuous solid lines are recordings of the downhole Amerada pressure gauge, and the dotted lines are the DMR records adjusted to the elevation of the Amerada gauge. Vertical Stress. As stated above, the principal earth stresses at Belridge are assumed to be oriented vertically and horizontally. SV was determined from formation-density-compensated (FDC) logs run in Wells B and D. Differences of less than 20 psi [140 kpa] were observed over the intervals of interest, so averages of measurements from these two wells were used to provide SV as a function of depth for all four wells. SPEFE P. 541^

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Section: Stress Directionsmentioning

confidence: 92%

Earth Stress Measurements in the South Belridge Oil Field, Kern County, California

Hansen

Purcell

1989

SPE Formation Evaluation

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“…It had been found that the orientation of the maximum strain observed during ASR Tests coincides with the orientation of the maximum horizontal stress as indicated by microseismic monitoring of hydraulic fracturing operations in conventional reservoirs (Lacy 1984). This has been attributed to microcracking (Lacy 1984;Teufel 1989;Holt & Kenter 1992) during the coring and core recovery processes, leading to anisotropy of elastic modulus in the horizontal plane.…”

Section: History and Objectivesmentioning

confidence: 86%

“…This has been attributed to microcracking (Lacy 1984;Teufel 1989;Holt & Kenter 1992) during the coring and core recovery processes, leading to anisotropy of elastic modulus in the horizontal plane. Recalling the conclusions drawn from early point load tests on core (Friedman & Logan 1970), it was recognized at BP that a method involving perturbation of the residual stress state of orientated core under normal laboratory conditions might be a far more practicable, reproducible and efficient means of determining the orientation of maximum horizontal stress.…”

Section: History and Objectivesmentioning

confidence: 99%

The nature of stress state: numerical and laboratory experiments and some field observations

Harper¹,

Hagan²

2015

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Simulations of the stress state in a region of discontinuously fractured rock revealed a strongly heterogeneous, self-organized stress distribution displaying internally balanced strains and corresponding energy storage. The explicit simulations follow a sequence which may be inferred to be irreversible and strongly dependent on interaction between fractures, implying a time-dependency. The state of the simulated reservoir displays a sensitivity characteristic of marginal stability. These numerical observations are compared with new and previous laboratory and field experiments spanning a wide range of size, stress magnitude and stress difference. The characteristics of the state of the simulated reservoir, laboratory samples and field behaviour are found to be consistent with the numerical predictions. A stress memory is evident. This supports previous conclusions that the concept of stress state as uniform, constant and resulting from averaged values of stresses or displacements, presently applied at remote and artificial boundaries, is grossly oversimplified and flawed.

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“…Vertical ellipsoid with two symmetric wings transmitting without end from the wellbore arranged opposite to the direction of slightest primary anxiety (Ilderton et al, 1996). The coveted arm compass of every crack is regularly around 300 ft tip-to-tip, and the sought stature speaks the truth 100-150 ft. Wells are thickly separated every 165 ft or less., making the learning of hydraulic break geometry orientation, length, stature, symmetry and profundity fundamental in deciding ideal right areas (Lacy, 1987).…”

Section: Introductionmentioning

confidence: 99%