In-Situ Stress Measurements at U. S. DOE's Multiwell Experiment Site, Mesaverde Group, Rifle, Colorado

Abstract

Summary Vertical distribution measurements of the minimumprincipal 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'sMultiwell Experiment (MWX) site have been made by conducting small-volume, hydraulic-fracture stress teststhrough perforations. Accurate, reproducible results wereobtained by conducting repeated injections in each zoneof interest with a specially designed pump system, modified high-resolution electronic equipment, and adownhole shut-off tool with a bottomhole pressure (BHP)transducer. Stress tests were conducted in marines and stones and shales as well as in coal, mudstone, andsandstone in a paludal depositional environment; these testsprovide 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 nearlylithostatic, with a fracture gradient greater than 1.0 psi/ft[23 kPa/m]. This indicates that these rocks do not behaveelastically and processes such as creep and possibly fracturing are the dominant mechanisms controlling the stressstate. Sandstones and siltstones have much lower stresses. with a fracture gradient of 0.85 to 0.9 psi/ft [19 to 20kPa/m]. Containment of hydraulic fractures would beexpected under these conditions. Only three data points wereobtained from the paludal interval; no significant stressdifferences were observed in the different lithologies. Introduction The vertical distribution of the minimum principalhorizontal in-situ stress, has a significant influenceon hydraulic fracture geometry. Perkins and Kern notedits importance with respect to fracture height, and Simonson et al demonstrated how to calculate fracture heightin a nonuniform, but symmetric, stress field. Laboratory and mineback experiments have provedthe effect of differences on fracture height, but, as yet, field experiments have not yielded conclusive results. This results primarily from the lack of detailedin-situ stress data and viable fracture height measurement techniques. In addition, few in-situ stress measurementshave been obtained in intervals where core is availableso that stress/rock-property correlations can be attempted. The present work on in-situ stress measurements is partof DOE's MWX program, which is being conducted inthe Piceance basin near Rifle, CO. In-situ stressmeasurements currently are planned throughout the entire4,000 ft [1200 m] of Mesaverde rocks encountered at thislocation, with particular emphasis on obtaining detailedstress 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, havebeen run. This paper presents the results of the initial series of in-situ stress tests, which were conducted at thebase of the Mesaverde in marine sandstones and shales and at various horizons in a paludal zone. These data willbe used to design hydraulic fracture treatments and aidin 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 Nolteand Smith, yields a reliable, reproducible estimate ofand typically is conducted in an interval soon tobe stimulated. These tests have been called "minifraes"because they use small volumes of fluid (500 to 10,000gal [2.0 to 40 M ]) compared with conventional fracture treatments. The technique used in this study also is calleda "minifrac, "although it uses much smaller pumped volumes (1 to 250 gal [0.004 to 1.0 m ]). Minifracsusually are conducted by injecting a small volume intothe formation, shutting in, and measuring the instantaneous shut-in pressure (ISIP). For openhole tests, several authors have discussed the technique'sdetails, and it is clear that when adequately conducted, the test yields an accurate, reproducible estimate ofand a somewhat less reliable estimate of the maximum horizontal in-situ stress,. For most oil and gas applications, however, it isimpossible or impractical to conduct these tests in anopenhole environment. Problems with hole stability, gaspressure, 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, andrandom perforation orientation effects. JPT p. 527