Lima, C. (Independent Researcher) | Lavorante, L. P. (Independent Researcher) | Williams, W. C. (Louisiana State University) | Beisl, C. (UFRJ-COPPE) | Reis, A. F. C. (Petrobras) | Carvalho, L. G. (Petrobras) | Moriss, M. (Paradigm)
ABSTRACT: This study proposes that a systematic comparison using integrated 3D visualization of all pertinent data (midplate seismicity, geological and geophysical variables) could help in identifying areas vulnerable to injection-induced seismicity in the North American plate. From similar studies of the South American plate in Brazil’s Potiguar basin, it is found that intraplate seismicity occurs at uplifted basin borders (areas over thin, hot, weaker lithosphere) where pre-existing faults are prone to be reactivated by small pressure perturbations. Conversely, central basins (areas over thick, cold, strong lithosphere) are not prone to seismicity. With this model we investigate Oklahoma (Ok) and North Dakota (ND), both intense areas of injection. ND activity, in the central basin, shows no induced seismicity. In contrast, Ok activity, in a regional-scale ravine in the uplifted basin border, has seen a 62.5-fold increase in recent seismicity. Modeling of the Ok region shows reactivation of pre-existing faults with injection pressures of 1.75 MPa (254 psi; 0.7ppg) between 2000-2200m depths, values that agree with wellhead injection pressure field data.
1. INTRODUCTION: THE PROBLEM
A huge increase of seismicity in the tectonically stable U.S. is put into evidence, if we examine the USGS Catalog, 2017 comparing the number of earthquakes of magnitude (Mw) greater or equal to 4 that occurred during 2000-2010 and 2010-2016. For this area, see Fig. 1, we jumped from an average of 6.2 events/yr, during 2000-2010, to an average of 28.8 events/yr, during 2010-2016, roughly a 5-fold increase. For Oklahoma, see Fig. 2, a 62.5-fold increase of seismicity has been observed when comparing these same two periods, including two major events (Mw 5.7, 2011; Mw 5.8, 2016). These recent increases are contemporaneous with the increase in shale production as shown in Figs. 1 and 2. In the stable midcontinent, a roughly 5-fold increase is observed in seismicity during 2010-2016. Again, the increase is contemporaneous with US shale production.
The sedimentary rocks cover about 70% of the Brazilian territory. Large sedimentary basins, from the Paleozoic to the early Cenozoic, present a wide variability of lithology, raging from coarse grained rocks as the conglomerates, to the fine grained siltstones and claystones. Geotechnical parameters (index properties, resistance to uniaxial and triaxial compression strength, deformability, disaggregation, slaking, drillability, erosion resistance, etc.) have been determined for the several rocks types, concerning various projects like roads, channels, dams, tunnels, etc. The tropical weathering has been considered a factor of great importance in the evolution of these rocks, due to the decrease of their resistance and deformability in some cases, and the lateritization and cementation, in others. This paper briefs information regarding the geotechnical properties of several sedimentary rocks in Brazil, as well as gives some guidelines to foresee their characterization related to strength, weathering and slaking. In order to be restricted to the subject itself, we will not observed the metamorphic rocks of low degree and the sedimentary rocks strengthened by silicification, such as the sandstones silicified by the basaltic flows.
Chitale, Vivek (BP Brazil Exploration) | Alabi, Gbenga (BP Brazil Exploration) | Kasten, Rob (BP Brazil Exploration) | Taylor, Alistair (BP Brazil Exploration) | Hoenmans, Paul (BP Brazil Exploration)
Textural and compositional heterogeneities present in the pre-salt carbonates in Campos Basin required deployment of a variety of modern technologies, techniques and approaches with respect to the petrophysical evaluation of cores as well as e-logs acquired from the newly drilled wells in these reservoirs. Experience gathered from these wells has been captured in this paper which presents a summary of our approach to dealing with the challenges posed by the pre-salt carbonates. Successes as well as limitations of the approach are pointed out. Value added by our approach for petrophysical formation evaluation of these carbonates is also discussed. This is expected to be useful for other similar carbonate exploration and appraisal projects.
The case history presented here highlights learning from the implementation of a comprehensive program of data acquisition and interpretation designed to characterize the complex carbonates of the Aptian Sag formation of the pre-salt play. Wells were drilled using oil based mud (OBM). Data acquisition consisted of conventional and rotary sidewall coring, and full suites of LWD and wireline logging. LWD was used to ensure availability of optimal logs from relatively un-invaded formations. The LWD dataset included current generation gamma, resistivity, density, porosity, sigma, elemental spectroscopy and NMR technologies. Wireline logs included spectral gamma, induction, density, neutron, sonic, elemental spectroscopy, latest generation NMR T1 and T2, borehole imaging (electrical and acoustic), rotary cores and pressure and fluid sampling surveys. Plugs were drilled from the conventional core at the wellsite immediately after coring and they were kept in preserved condition before analysis. During coring tracers were added to the mud and their concentrations in the core as well as muds were determined later in order to characterize invasion by both oil and water phases in the OBM.
Evaluation of e-logs and core samples shows that using the above mentioned combination of technologies and approaches helps to greatly reduce the uncertainty in the evaluation of pore space and the solid matrix in the Sag carbonates enhancing our understanding of the following petrophysically critical properties of the pre-salt carbonates: mineralogical and geochemical variations in the solid matrix affecting the grain densities, textural heterogeneity in the formation causing spatial anisotropy in terms of porosity, pore size distribution and permeability, and presence of fractures. We found that a certain degree of experimentation was necessary to deal with the challenges posed by the complexities of the pre-salt carbonate reservoir.
The reservoir characterisation of lacustrine carbonates is currently challenging, as historically they are not common as major carbonate reservoirs and therefore have been less studied in terms of reservoir characterization. The giant discoveries of the Pre-Salt of the South Atlantic Margin in Brazil have introduced lacustrine carbonates as a major target for exploration and development and are driving new research.
Lacustrine carbonates are formed in lakes which are particularly sensitive to salinity, pH, water level and sediment supply variations. This variability results in a wide range of primary fabric types and associated porosity types, likely formed by the interaction of microbial and chemical processes. Lacustrine grainstones can be formed from spherulites or coquinas and stromatolites and stromatolite bioherms can form cavities and laminated pore systems. Shrub morphologies are found that can form complex pore networks. These fabric types can be affected by a series of diagenetic processes that can both enhance and occlude porosity.
In this paper, we review how porosity in these systems may be characterised in thin sections and using micro-CT and how permeability and resistivity might be determined in the laboratory or by 3-D modelling. We also address how rock types may be determined by analysis of capillary pressure data, to determine relative permeabilities for different rock types. This paper addresses some classical reservoir characterisation issues that are likely to be important in these reservoirs. The approach considered here emphasises an integrated link between petrography, facies analysis (outcrop studies) and porosity description in a petrophysically-based rock typing framework appropriate for geomodelling, upscaling and calibration with dynamic data. This reservoir characterisation workflow methodology is applicable to all carbonates, however, it is recognized that lacustrine carbonates present some special challenges.