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ABSTRACT ABSTRACT: Wellbore stability in shales is very much influenced by the type of drilling fluid. The pore pressure generated during drilling and its diffusion into the shale are major contributors to both short and long term wellbore stability. The membrane effect and the subsequent osmotic pressure developed around the well, play an important role in the force balance that may destabilize the rock. The present paper focuses on assessing shale behavior when exposed to some real water-based drilling fluids. The physical-chemical aspects of the shale-fluid interaction are evaluated, in particular, in relation to osmotic pressure and membrane effects developments. Rock-fluid interaction tests, using shale samples collected from Campos Basin, Brazil, and the Norwegian North Sea, confirmed the efficiency of the designed equipment in carrying out experiments with highly viscous, water-based fluids. The experiments showed that the fluid pressure transmission behavior, for the shales and fluids used, does not differentiate between the viscous water-based fluid and pure water. The development of osmotic pressure suggests that the shale reflection coefficient is more influenced by its pore size distribution than by its mineralogy. INTRODUCTION Muniz et al. (2004), [1], described an apparatus designed to evaluate shale-drilling fluid interaction. The testing set-up is such that both hydraulic and ionic gradients can be imposed on to the shale sample, in order to estimate shale permeability, its coefficient of reflectivity (membrane efficiency) as well as ionic diffusion coefficient. These parameters should be used as input in borehole stability programs that consider the physical-chemical interactions. The tests described by [1] were carried out using saline, aqueous solution to mimic water-based fluids. However, as pointed out in [1], the ideal drilling fluid concerning stability of wells must keep the confining effective stresses around the well high enough to preclude rock failure. This can be achieved by at least three distinct manners. Firstly, by avoiding pore pressure increase due to fluid penetration, through the use high-entry pressure fluids, i.e., oil-based-like type of fluid. Secondly, by playing with the osmotic effects caused by water-based, saline fluids. Thirdly, by the use of invert-emulsion fluids, [2], that combine the two previous mechanisms. Therefore, it is important to understand how real drilling fluids, not pure saline, aqueous solutions, interact with shales and which shale and fluid parameters are relevant to describe the process. Four pressure transmission tests were designed in order to contribute to such an understanding. Three shales (two from Campos Basin, offshore Brazil, and one from the Norwegian North Sea) with different pore structure and mineralogy were used. The tests were carried out in order to evaluate both the transmission pressure and the osmotic potential of a water-based mud. During the pressure transmission phase, the difference in pressure transmission between pure aqueous solution and the water-based mud consisting of xantam gum, 7.13 kg/m3 (2.5 lb/bbl), and HPA, 17.12 kg/m3 (6.0 lb/bbl) was investigated. For the ion diffusion phase, 40 % w/w of sodium formate was added in the water-based mud.
- South America > Brazil (1.00)
- Europe > Norway > North Sea (0.94)
ABSTRACT: This paper describes CU ? consolidated undrained - triaxial tests to determine the mechanical properties of shale cores from deep boreholes in the North Sea. The tests carried out at CENPES´s Rock Mechanics Laboratory employ a geotextile side drain to accelerate the phases of saturation, consolidation and equilibrium of pore pressure during undrained axial loading. The saturation and consolidation phases are carried out under elevated backpressure. A simulated pore fluid is used in the pore pressure line. In order to avoid damage to the equipment caused by corrosion, a new top cap adapted with a small pressure transducer was developed. The undrained shear phase is conducted at slow axial strain rate that is selected to allow adequate redistribution of pore pressure throughout the samples. Five consolidated undrained triaxial tests were conducted under a range of confining pressure from 9 to 20 MPa with 5 MPa of backpressure. The strength behavior for this shale can be described using a linear Mohr-Coulomb strength criterion with effective cohesion of 7,2 MPa and an effective angle of internal friction of 10,6°. The mainly contribution of the used methodology is the reduction of the total testing time, optimizing the usage of the triaxial equipment. INTRODUCTION Shales are sedimentary rocks resultant from the compaction of clay rich sediments and constitute the most frequent type of rocks encountered during drilling for oil and gas. The evaluation of wellbore stability in shales requires the knowledge, amongst others factors, of its stress-strain behavior as well as its shear strength properties. Stress-strain tests in shales are not common to be executed due to the difficulties in obtaining good quality samples, specially the high costs associated with coring and the lack of a well-established testing procedures for these materials. The Norwegian experience in testing shales from the North Sea has been reported by Remvik [1] and Horsrud et al. [2]. Steiger and Leung [3,4] discuss the experience accumulated in testing shales from the Gulf of Mexico and Wu et al. [5] present the testing procedures for shales from the Australian Shelf. The Brazilian experience in testing shales extracted from deep layers offshore Brazil is small and is concentrated mainly in Petrobras?s rock testing program and in the research projects conducted under the sponsorship of Petrobras in a joint effort between Petrobras Research Center (CENPES) and the Group of Petroleum Technology and Engineering (GTEP acronym in Portuguese) from the Civil Engineering Department of the Pontifical Catholic University of Rio de Janeiro (PUC Rio). In 1996 a large testing campaign was launched aiming at defining a proper methodology for executing consolidated, undrained triaxial tests in Brazilian shale. Muniz [6] describes the methodology developed for carrying out undrained triaxial tests in shales that was used in this present paper. Emphasis is given to the use of a lateral drain as a mean to equalize pore pressure within the sample, to the high level of backpressure used for sample saturation and to the very high consolidation rate achieved during the experiments. The present paper shows the results and analyses of 5 CU (consolidation stage under isotropic state of stress and shear stage under undrained conditions) triaxial tests carried out on samples from the Norwegian North Sea, provided by Statoil, extracted from depths over 2000 meters. These tests were performed at the Rock Mechanics Laboratory at CENPES (Petrobras Research Center).
- Europe > United Kingdom > North Sea (1.00)
- Europe > Norway > North Sea (1.00)
- Europe > Netherlands > North Sea (1.00)
- (2 more...)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > South America Government > Brazil Government (0.86)