In-Situ Stresses in Low-Permeability, Nonmarine Rocks

Abstract

Summary In-situ stress measurements have been performed over a 2,500-ft [760-m] interval in the nonmarine section of the Cretaceous Mesaverde in the Piceance basin in Colorado. These measurements included 52 hydraulic fracture measurements of the minimum in-situ stress and 22 anelastic strain recovery (ASR) measurements. Stress data obtained in sandstones, shales, mudstones, siltstones, and coals show the effect of lithology on the magnitudes of the stresses. Introduction Many oil and gas production and completion activities depend on the in-situ stresses in and around the reservoir rock, and the degree of success of these activities can often be influenced by determination of the stresses and their inclusion in the design process. Some important examples include hydraulic fracture growth, proppant crushing in hydraulic fractures, fracturing during waterflooding, wellbore stability, and the behavior of natural fractures during any process in which the pore pressure and/or stresses are altered. Knowledge of the stress field at depth does not come easily, however. Calculations of the stresses are relatively crude because the three-dimensional stress field depends on a complex history of properties and loading conditions and may include anelastic, plastic, and fracturing episodes. While much of the effect of this past history may be negligible in many cases, stresses may also be locked in sufficiently so that the total history must be known. Additionally, we would also need to know other conditions, such as drained vs. undrained behavior of the pore fluids, whether the tectonic loads should be considered stress or strain boundary conditions (important for layered media), the history of diagenesis, and details of the constitutive model(s) of the bulk rock. In all but the simplest cases, we will have difficulty performing credible stress calculations. The previous discussion suggests that stress measurement is the preferred option, and much research has been performed to develop usable stress measurement techniques that can be applied at depth in a wellbore. These includethe well-known hydraulic fracturing technique;various core analyses, such as ASR, differential strain curve analysis (DSCA), and differential wave velocity analysis;wellbore condition logs (televiewers and calipers) to examine eccentricity and breakouts;hydraulic fracture diagnostics to estimate the stress field orientation; andother ideas. The subject of this paper is two-fold:a compendium of all in-situ stress measurements conducted in nonmarine rocks at the Multiwell Experiment (MWX) in the Piceance basin of Colorado anda critical analysis of the techniques, results, and implicat ions. This study builds on many previous stress measurements and analyses at this site, many of which have been in marine rocks. MWX Site The MWX25 is an experiment funded by the U.S. DOE to characterize low-permeability reservoirs in the western U.S. and to test technology for the production of natural gas from these marginal resources. The site is located in the Piceance basin in west-central Colorado near the town of Rifle. The facility consists of three closely spaced wells (100 to 220 ft [30 to 67 m] apart) painstakingly char-acterized by the recovery and analysis of more than 4,100 ft [ 125m] of core, advanced logging programs, detailed well testing, frequent stress testing, and highly instrumented stimulation experiments. The Mesaverde group, the tight sand target, is between the depths of 4,100 and 8,200 ft [ 1250 and 2500 m] at this location.