In-Situ Stress Determination Based Upon Borehole Imaging and Rock-Sample Analyses: A Comparison With Anelastic Strain Recovery (ASR), Acoustic Velocity, and Acoustic Tomography

Abstract

Abstract The orientation of the horizontal in-situ stresses in an oil well at the Potiguar Basin, Northeast of Brazil, was estimated through three different methods: Anelastic Strain Recovery Test - ASR-3D, wellbore Electrical Micro Imaging - EMI, and acoustic velocities anisotropy - VELAN. A fourth and very promising method for acquiring in-situ stress orientation from rock cores, acoustic tomography, is also presented in this paper. The goal of the work was to compare methods dealing with different sources of data, in order to identify a more reliable, though simple and cheap procedure for inferring the in-situ stress orientation. The three methods had very similar results, finding the maximum horizontal in-situ stress at N42,6E (ASR method); N45E (EMI method); and N42E (VELAN method). Introduction There are several applications in the oil industry for knowing the in-situ stress field, like estimating reserves-in-place[1], helping to define a more efficient drainage mesh[2], optimizing hydraulic fracturing[3] and wellbore stability[4]. Measuring the in-situ stress deep underground, and sometimes at water depths nowadays reaching more then 3,000 m, is not an easy task. The usual procedure has been to compare different methods, in order to get a reliable estimative of the stress orientation and magnitude. Several methods are available in the literature, ranging from more abroad to more punctual, like the stress world map, seismic observations, logging and rock sample testing. Four methods are presented in this paper, together with a field case where three of these methods have been compared. According to early researches on the stress history computed from rock samples, the principal stress orientation can be directly computed by assuming the same orientation of the principal strains[5]. This is the basics for the very well known Anelastic Strain Recovery - ASR method, were rock samples are monitored immediately after coring, showing expansion due to the stress relief and/or contractions due to porepressure diffusion[6]. The higher strain direction being associated to the orientation of the higher principal in-situ stress. Imaging borehole allows oil and gas operators to visualize detailed geological features with a good degree of reality. Operators can use it to evaluate sedimentary features, textural characteristics, thin beds and fracture orientation with a high level of precision. Drilling-induced fractures can be associated to the pre-existing stress field, since the drilling fluid pressure will open a fracture easier towards the least in-situ stress, perpendicular to the direction of the maximum in-situ stress. Thus, whenever visible, the simple yet precise identification of these fractures at the wellbore wall will provide a clue for the in-situ stress orientation. The stress relief after coring leaves in the rock a permanent anisotropic set of microcracks, which will slow down acoustic propagation of elastic compressional - P waves, analyzed bythe acoustic velocity anisotropy - VELAN method. The direction of maximum time travel, which coincides with the direction of the higher crack opening density, is the most probable orientation for the maximum in-situ stress[7]. Ultrasonic tomography of core samples may be very useful for identifying rock petrophysical attributes, such as mineral composition, presence of heterogeneities or discontinuities, patch saturation and internal distribution of stresses. In this method, the large number of transducers provide a more realistic 3-D characteristic. The several different planes of velocity anisotropy are then analyzed through a sophisticated computer program. As well as the VELAN method, the slowest P-wave direction will be associated to the maximum in-situ stress.