Lawrence W. Teufel

Lithologic and Structural Controls on Natural Fracture Distribution and Behavior Within the Lisburne Group, Northeastern Brooks Range and North Slope Subsurface, Alaska

The Carboniferous Lisburne Group of northern Alaska has been deformed into a variety of map-scale structures in both compressional and extensional structural settings, thus providing a series of natural experiments for observing the formation, distribution, and behavior of fractures in this thick carbonate unit. Two fracture sets dominate the Lisburne Group carbonates of the North Slope subsurface and the nearby northeastern Brooks Range fold and thrust belt. North-northwest-striking regional extension fractures probably formed in front of the northeastern Brooks Range fold and thrust belt. In the North Slope subsurface, this fracture set overprints east-northeast-striking fractures related to earlier extensional deformation; in contrast, in the fold and thrust belt, the north-northwest-striking fracture set is overprinted by younger east-northeast-striking fractures related to subsequent contractional deformation. Lithology is the primary control on the fracture density of both sets. In mildly deformed Lisburne ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received February 20, 1996; revised manuscript received November 25, 1996; final acceptance June 5, 1997. 2Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775. 3Sandia National Laboratories, MS 0705, Albuquerque, New Mexico, 87185. 4Department of Petroleum & Natural Gas Engineering, New Mexico Institute of Mining & Technology, Socorro, New Mexico 87801, and Sandla National Laboratories, MS 0705, Albuquerque, New Mexico, 87185. This study was supported by a Department of Energy subcontract administered by Sandia National Laboratories. Additional support was provided by ARCO Alaska, BP Alaska, Chevron, Exxon, Mobil, and Japan National Oil Company. We would like to thank ARCO Alaska and BP Alaska for giving us permission to view selected Lisburne Group cores, W. Wallace for helpful discussions on deformational styles of detachment folds, A. J. Mansure for help in interpreting the interference tests, and W. Wallace, K. Biddle, W. Belfield, N. Hurley, and J. Kelley for helpful reviews of the manuscript.

In situ stress measurements at Rainier Mesa, Nevada Test Site ― influence of topography and lithology on the stress state in tuff

Abstract A large number of in situ stress measurements have been conducted in Rainier Mesa at the Nevada Test Site. Most of the measurements were made using the hydraulic fracture technique, but some comparisons with overcoming were also performed. Many of the hydraulic fractures were mined out and their behaviour was directly observed. The results show that horizontal in situ stresses in the mesa depend upon the depth, material properties, topography, bedding and faults. Large stress changes occur over distances much less than a metre, particularly near interfaces and faults. Bedding and topography were found to reorient stresses by as much as 45°. This paper gives a case study of work at the Nevada Test Site and documents the variations in the stress field due to these geologic features.

Natural fracture characteristics and effects

Natural fractures of one type or another are typical of hydrocarbon reservoirs in virtually all structural settings, from tightly folded strata to otherwise undeformed, flat‐lying formations. However, since different structures impose fracture‐forming stresses onto the strata in different ways, significantly different fracture intensities and patterns may be produced in different settings. Moreover, not all fractures of a given set are equal in length, aperture, or height (and therefore in their effect on permeability or seismic signal), even if they formed in the same stress regime: The range of apertures or lengths within a fracture set, for example, may consist of numerous narrow and/or short fractures and a few wide, long ones. This paper briefly discusses “regional” fractures, fractures that are common as relatively regular, extensive, parallel to subparallel features over wide areas within relatively undeformed strata (Figure 1).

Laboratory measurements of the effective-stress law for carbonate rocks under deformation

The behavior of rocks under the combined effects of confining stress and pore pressure is an important issue for any in situ petroleum process. In order to simplify the difficulties in dealing with two independent parameters, it is customary to introduce an effective-stress law which relates a net, or effective, stress to some combination of confining stress and pore pressure. This report documents laboratory tests of mechanical properties of five carbonate rocks.

Results of the multiwell experiment in situ stresses, natural fractures, and other geological controls on reservoirs

Hundreds of millions of cubic meters of natural gas are locked up in low-permeability, natural gas reservoirs. The Multiwell Experiment (MWX) was designed to characterize such reservoirs, typical of much of the western United States, and to assess and develop a technology for the production of this unconventional resource. Flow-rate tests of the MWX reservoirs indicate a system permeability that is several orders of magnitude higher than laboratory permeability measurements made on matrix-rock sandstones. This enhanced permeability is caused by natural fractures. The single set of fractures present in the reservoirs provides a significant permeability anisotropy that is aligned with the maximum in situ horizontal stress. Hydraulic fractures therefore form parallel to the natural fractures and are consequently an inefficient mechanism for stimulation. Successful stimulation may be possible by perturbing the local stress field with a large hydraulic fracture in one well so that a second hydraulic fracture in an offset well propagates transverse to the natural fracture permeability trend.

Determination of in situ stress from anelastic strain recovery measurements of oriented core: comparison to hydraulic fracture stress measurements in the Rollins Sandstone, Piceance Basin, Colorado

A method to determine in situ stress directions and magnitudes from anelastic strain recovery measurements of oriented core has been used to determine the principal horizontal in situ stresses in the Rollins Sandstone in a deep well near Rifle, Colorado. The principal horizontal stress directions were determined directly from the principal horizontal strain recovery directions. The principal horizontal stress magnitudes were calculated from the principal strain recovery magnitudes, overburden stress, and Poissons ratio of the rock using a viscoelastic model. The accuracy of the in situ stress directions and magnitudes determined from anelastic strain recovery measurements was substantiated by a direct comparison with open-hole hydraulic fracture stress measurements. The anelastic strain recovery method predicted the magnitudes of the maximum and minimum horizontal principal stresses to be 54.5 MPa and 51.9 MPa, respectively; with an azimuth of N63/sup 0/W +- 8/sup 0/ for the maximum horizontal stress. The hydraulic fracture stress measurements yielded maximum and minimum horizontal principal stress magnitudes of 49.6 MPa and 46.8 MPa, respectively. The azimuth of the maximum horizontal stress ranged from N50/sup 0/W to N70/sup 0/W. 17 references, 2 figures, 4 tables.

Effect of Stress and Pressure on Gas Flow Through Natural Fractures

Gas conductivities of narrow natural fractures in sandstone and chalk were measured under varying stress and pore pressure conditions and showed a decrease in conductivity with increasing net stress. Natural fractures in mudstones exhibited continuously decreasing conductivity upon application of stress, so that correlatable conductivity data could not be obtained. Effective-stress-law behavior for the sandstone and chalk fractures were examined, giving {alpha} values in the range of 0.8--1.06, where {alpha} is the parameter in the effective-stress law, {sigma} - {alpha}P. The value of {alpha} for the fracture in chalk was nearly constant, but the values for the fractures in sandstone tended to decrease with increasing stress. Transition Reynoldss numbers and turbulence factors for flow through the chalk and sandstone fractures were determined, yielding turbulence factors ranging from 6.0--20 {times} 10{sup 6} ft{sup {minus}1} (2.0--6.6 {times} 10{sup {minus}5} cm{sup {minus}1}) for differently stressed fractures. The entire flow behavior of these natural fractures, including conductivity, effective-stress law, and turbulence, is controlled by stress and pore pressure. As a result, pressure depletion during production will significantly change the productivity of a reservoir with similar natural fractures. 28 refs., 14 figs., 4 tabs.

An Assessment of the Factors Affecting Hydraulic Fracture Containment in Layered Rock: Observations from a Mineback Experiment

In situ experiments, which were accessible for direct observation by mine-back, have been conducted to determine the effect that geologic discontinuities, elastic properties, and in situ stress differences have on hydraulic fracture propagation and the resultant overall fracture geometry. Vertical fractures were observed to terminate only in regions of high minimum horizontal in situ stress. Fracture growth into a higher (by a factor of 5 to 15) elastic modulus region was preferred to propagation into a region of higher (by a factor of 2) stress. Geologic discontinuities did not arrest the lateral or vertical propagation of hydraulic fractures. However, hydraulic fracture growth was affected by discontinuities, because fracture-fluid leakoff occurred along intersecting discontinuitiDS which the hydraulic fractures crossed, and thereby reduced the total extent of the hydraulic fractures. The results of the mineback experiment clearly indicate that differences in the minimum horizontal in situ stress between the reservoir rock and adjacent layers is the most critical factor affecting hydraulic fracture containment.

A Model for Fracture Genesis--Application to Mesaverde Group, Piceance Creek Basin, Colorado: ABSTRACT

Natural fractures play an important role in determining gas production from the low-permeability reservoirs of the Mesaverde Group in the Piceance Creek basin, Colorado. The importance of natural fractures is evident from the number of natural fractures observed in core and from the high in-situ permeabilities measured in well tests as compared to the low permeabilities measured in core. An understanding of the natural fracture systems requires knowledge of variations in the state of stress and changes in the physical and mechanical properties of the different sedimentary layers during the evolution of the basin. Geologic processes such as burial, diagenesis, tectonics, uplift, and erosion, and their resultant effects on the overburden, pore pressure, temperature, and str in were included in an elastic-plastic model to approximate the stress history of the basin. These data, coupled with an extended von Mises failure criterion derived from laboratory experiments of the rocks in question, were used to predict the relative time and type of fracturing, and the lithologic layers in which a fracture was likely to occur. Observations of fractures in 4,200 ft (1,280 m) of core (1,200 ft, 365 m, of oriented core) from the Mesaverde Group taken from the United States Department of Energys 3 closely spaced wells near Rifle, Colorado, have been used to document the genesis of natural fractures and substantiate the model results. Empirical information such as the present state of in-situ stress determined from hydraulic fracture stress tests and anelastic strain rec very measurements of oriented core, paleostrain directions and magnitudes determined from analysis of calcite twin lamallae, and current temperature and pore pressure provided data as well as checks on the accuracy of the model. End_of_Article - Last_Page 534------------