Integrated Horizontal Well Stability Design in Potiguar Basin

Abstract

Problems related to wellbore stability can increase drilling and completion costs. At the limit it can be impossible to drill a well because of wellbore mechanical instability. Several wellbore stability simulators can be found in the literature. Some of them present very simplified analytical solutions while others are numerically complex. Besides choosing the simulator, it is necessary to know the elastic and strength properties of the rock, the in-situ stress field, rock porosity and permeability and original pore pressure. Determination of some of these input data are costly and sometimes impossible. This paper presents the data gathering and design of a horizontal well in the Potiguar Basin (fig. 1) in Northeast Brazil. The main goal of this work is the determination of the mud density to drill a horizontal well, safely and economically viable, in order to avoid wellbore collapse (inferior limit) and formation breakdown (superior limit). Introduction This paper presents an integrated wellbore stability design for drilling a horizontal well in the Potiguar Basin. An integrated wellbore stability analysis means data gathering and mechanical stability numerical simulation. Underground rocks are always under a compressive stress field. When drilling a well, an amount of material is removed and consequently the wellbore neighborhood needs to support the load that was supported by the removed material. This new stress state may cause the wellbore rupture depending on the mud density and formation strength parameters. Rock behavior is totally different if it is submitted to compressive or tensile loading. Because of this, it is necessary to consider two different failure criteria to properly represent rock collapse behavior (compressive stress state) and another to represent formation breakdown (tensile stress state). These two criteria are presented in Appendix A. Field data and lab tests have provided the in-situ stress field and rock mechanical properties for the horizontal well stability analysis presented in this paper. Fracture propagation and pressure decline were analyzed from step rate test (SRT) allowing the stress field evaluation. Unfortunately a minifrac was not scheduled for this field. Elastic and strength parameters were determined from triaxial (drained) and uniaxial compressive tests done by Rock Mechanics Laboratory at Petrobras Research Center (CENPES). Fluid properties, rock porosity and permeability, were obtained through lab tests. The bottom hole static pressure was determined in field. A Finite Element Method (FEM) simulator (AEEPECD) was developed at CENPES for mechanical stability analysis in plane strain problems - The material constitutive law was considered elastic-plastic. MTOOL and MVIEW were used as pre and post-processors, respectively. Input Data The in-situ stress field was determined from field tests while rock properties were obtained through lab tests. Permo-porous and fluid properties, shown in Table 1, were taken from the reservoir database. In-situ stress magnitudes determination. Two basic assumptions were made for determining the in-situ stress field:The vertical stress is one of the principal stresses;The minimum in-situ horizontal stress magnitude can be given by the fracture closure pressure and its direction is perpendicular to the fracture propagation direction. The formation was fractured at a depth of 4163 ft using brine. The overburden gradient (1.03 psi) was determined from logs while the horizontal stresses were determined using the SRT. Four cycles were run with the SRT (Fig. 2) but only the first and the third ones were utilized for computing the horizontal stresses. The fourth cycle had operational problems.