LWD solutions are a cost-effective means of evaluating carbonate reservoirs
Carbonate reservoirs often show a high degree of geological and petrophysical heterogeneity. The variations stem from differences in localized depositional environment and diagenesis, ultimately dictating the poro-perm, wettability and overall hydrodynamic behavior of the reservoir at field scale. Secondary porosity and fractures are often the objective target when designing well plans, since their presence dramatically improves poro-perm. Productivity can often be improved by drilling horizontal wells to intersect more of these fractures and increase the overall sandface. Similarly, Halliburton’s 20/20™ Acid stimulation services help improve surface contact area, further enhancing production.
Water saturation (Sw) can be derived using a combination of conventional LWD datasets and can then be spatially referenced within a DecisionSpace® geological model so the proximity to either free water level (FWL) or waterflood front is understood (see figure 1). By understanding the fluid distribution within the interwell space, the multistage stimulation designs can be improved, increasing productivity and avoiding early water breakthrough. In situations where water saturation calculation from conventional quad-combo logs is difficult, nuclear magnetic resonance (MRIL-WD) data is often useful. MRIL™ can be used either as a stand-alone porosity, permeability and saturation solution to compare against conventional log interpretations, or it
can be used as part of a more holistic petrophysical interpretation to evaluate wetting phases and low-resistivity pay. The combination of MRIL and XBAT™ acoustic measurements has frequently been used for robust porosity evaluation in countries where nuclear source transportation and licensing are costly1. An example of this type of multi-sensor interpretation in carbonate reservoirs is shown in Figure 2.
Carbonate reservoir productivity is often linked to natural fracture frequency. Zones that show significant amounts of natural fractures often exhibit enhanced productivity; however, these zones can also act as unwanted conduits for acid stimulation treatments. By mapping natural fractures using micro-imaging tools such as Halliburton’s AFR™, carbonate reservoirs can be characterized based on their secondary porosity, fault and fracture frequency2. The mapped fault and fracture zones can then be correlated to geophysical seismic attribute volumes—such as coherency and edge enhancement—within DecisionSpace providing greater insight into the overall well placement.
During the course of drilling horizontal wells in mature carbonate fields, large variations in pore pressure are often encountered. Hydrodynamic aquifer drive systems and injection wells frequently cause large pressure differentials associated with tilted fluid contacts, pressure ramps in depleted zones, flow channelling in fractures and reservoir compartmentalization. GeoTap formation pressure testing tools provide the critical pore pressure data needed to drill to TD safely. Smart drawdown technology and on-the-fly rate selection via the GeoSpan downlink communications allow quick testing even in low milliDarcy carbonates. The horizontal formation pressure data, together with the matrix permeability estimation, can be captured within the integrated operation and further used to optimize the stimulation pumping program.
1Sourceless Porosity and Anisotropy While Drilling Using Nuclear Magnetic Resonance and Azimuthal Acoustic LWD Drilling Technology: A Case History. SPE-175565. S. Azim, H. Turky, N. Thakuria, S. Khesroh, Kuwait Oil Company. A. Abdulkarim, O. Saukova, M. Samie, A. Aki, Halliburton.
2Reservoir Characterization, Fracture Mapping and Well Placement Using a Suite of Logging-While-Drilling Images With Multiple Resolutions in a Marginal Middle East Carbonate Reservoir. SPE-148089. K. Al Daghar, T. Ihab, R. Sayed, A. Abdelaal, L. Ramos, ADCO. R. Chemali, A. Aki, S. Azzam, O. Razak, Halliburton.
3Waterflood Conformance Study For a Carbonate Reservoir. SPE-111363. K. Al Anzi, R. Kumar, T. Al Rashidi, Kuwait Oil Company. J. Cumming, H. McKean, C. McKie, Halliburton.
4Optimal FE Acquisition in a Complex Carbonate Reservoir: A Case Study on the Karachaganak Field, Kazakhstan. SPWLA 49th Annual Conference. S. McCalmont, S. Chittick, BG Group. R. Nurgaliev, Karachaganak Petroleum. J. Russo, Chevron. R. Deady, J. Market, J. Wilson, P. Sands, E. Jaunis, Halliburton.