In-Situ Stress Measurements at U. S. DOE's Multiwell Experiment Site, Mesaverde Group, Rifle, Colorado
- Norman R. Warpinski (CER Corp.) | Paul Branagan (CER Corp.) | Roy Wilmer (CER Corp.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- March 1985
- Document Type
- Journal Paper
- 527 - 536
- 1985. Society of Petroleum Engineers
- 2.2.2 Perforating, 1.6.9 Coring, Fishing, 5.4.2 Gas Injection Methods, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 1.14 Casing and Cementing, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.3 Rock properties, 5.6.1 Open hole/cased hole log analysis, 2.5.1 Fracture design and containment
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Vertical distribution measurements of the minimum principal in-situ stress in the lower Mesaverde group principal in-situ stress in the lower Mesaverde group (7,300- to 8,100-ft [2225- to 2470-m] depth) at the U.S. DOE's Multiwell Experiment (MWX) site have been made by conducting small-volume, hydraulic-fracture stress tests through perforations. Accurate, reproducible results were obtained by conducting repeated injections in each zone of interest with a specially designed pump system, modified high-resolution electronic equipment, and a downhole shut-off tool with a bottomhole pressure (BHP) transducer. Stress tests were conducted in marine sandstones and shales as well as in coal, mudstone, and sandstone in a paludal depositional environment; these tests provide a detailed stress distribution in this region. provide a detailed stress distribution in this region. The stress magnitudes were found to depend on lithology. Marine shales above and below the blanket sands have large horizontal stresses that are nearly lithostatic, with a fracture gradient greater than 1.0 psi/ft [23 kPa/m]. This indicates that these rocks do not behave elastically and processes such as creep and possibly fracturing are the dominant mechanisms controlling the stress state. Sandstones and siltstones have much lower stresses. with a fracture gradient of 0.85 to 0.9 psi/ft [19 to 20 kPa/m]. Containment of hydraulic fractures would be expected under these conditions. Only three data points were obtained from the paludal interval; no significant stress differences were observed in the different lithologies.
The vertical distribution of the minimum principal horizontal in-situ stress, , has a significant influence on hydraulic fracture geometry. Perkins and Kern noted its importance with respect to fracture height, and Simonson et al demonstrated how to calculate fracture height in a nonuniform, but symmetric, stress field. Laboratory and mineback experiments have proved the effect of differences on fracture height, but, as yet, field experiments have not yielded conclusive results. This results primarily from the lack of detailed in-situ stress data and viable fracture height measurement techniques. In addition, few in-situ stress measurements have been obtained in intervals where core is available so that stress/rock-property correlations can be attempted.
The present work on in-situ stress measurements is part of DOE's MWX program, which is being conducted in the Piceance basin near Rifle, CO. In-situ stress measurements currently are planned throughout the entire 4,000 ft [1200 m] of Mesaverde rocks encountered at this location, with particular emphasis on obtaining detailed stress measurements around formations to be stimulated. Over 4,000 ft [ 1200 m] of core have been obtained from three closely spaced wells (130 to 180 ft [40 to 65 m]) so an abundance of core data is available. Complete conventional log suites, as well as various advanced and experimental logs including the long-space sonic logs, have been run. This paper presents the results of the initial series of in-situ stress tests, which were conducted at the base of the Mesaverde in marine sandstones and shales and at various horizons in a paludal zone. These data will be used to design hydraulic fracture treatments and aid in the analysis of postfracture performance.
In-Situ Stress Measurements
At present, the only reliable method of obtaining distribution is by measurement with small-volume hydraulic fractures. Two techniques are currently in use. The step-rate/flowback procedure, pioneered by Nolte and Smith, yields a reliable, reproducible estimate of and typically is conducted in an interval soon to be stimulated. These tests have been called "minifraes" because they use small volumes of fluid (500 to 10,000 gal [2.0 to 40 M ]) compared with conventional fracture treatments. The technique used in this study also is called a "minifrac," although it uses much smaller pumped volumes (1 to 250 gal [0.004 to 1.0 m ]). Minifracs usually are conducted by injecting a small volume into the formation, shutting in, and measuring the instantaneous shut-in pressure (ISIP). For openhole tests, several authors have discussed the technique's details, and it is clear that when adequately conducted, the test yields an accurate, reproducible estimate of and a somewhat less reliable estimate of the maximum horizontal in-situ stress, . For most oil and gas applications, however, it is impossible or impractical to conduct these tests in an openhole environment. Problems with hole stability, gas pressure, cementing, and cost factors usually require that pressure, cementing, and cost factors usually require that the tests be conducted in cased holes through perforations. This causes additional complications because of casing, cement annulus, explosive perforation damage, and random perforation orientation effects.
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