Unconventional approach to resolving primary and secondary porosity in Gulf carbonates from conventional logs and borehole images

Abstract

fax 01-972-952-9435. AbstractTraditionally, downhole porosity measurements are made through nuclear/acoustic logs (both wireline and logging-while-drilling) and cores.In homogeneous carbonates, log porosity is generally a good substitute for core data. However, in heterogeneous carbonates with complex pore systems resulting from leaching/dissolution, cementation, and dolomitization, discrepancies exist between log-and core-derived porosity. Moreover, thin-bedded layers of high and low porosity, which are found to have a pronounced effect on reservoir production, are routinely unidentifiable on conventional logs because of their limited vertical resolution capabilities (2-3 feet).An innovative methodology has been developed to generate a porosity log with a minimum vertical resolution of 0.2 inch that also takes into account the azimuthal heterogeneities of the carbonate reservoir exposed in a wellbore. To achieve the high-resolution porosity log and entire porosity spectrum around the well, borehole images are used as external constraints after calibration with log porosity and resistivity. This methodology has significant applications in carbonate reservoir characterization. First, the identification of thin intervals of very high (related to high permeability) and low (dense, stylolitic) porosity is possible. Second, vuggy and moldic porosity can be quantified, and third, carbonate facies and reservoir rock types can be characterized. Several case studies from the U.Cretaceous I L. Cretaceous reservoirs and Upper Thamama Reservoirs highlight the effectiveness of the methodology to characterize porosity heterogeneity. Known porosity variations from core and mercury injection data confirm the heterogeneity demonstrated on the high-resolution porosity log. Since most carbonate reservoirs exhibit considerable porosity heterogeneity, this methodology has significant potential application to improve reservoir characterization in many areas.

An Integrated Approach for Formation Micro-Image Rock Typing Based on Petrography Data: A Case Study in Shallow Marine Carbonates

“…This method locates the threshold at a user-defined fixed percentage (default value: 15%) above the mean porosity. Core observations and measurements provide a way to calibrate this value [27,37]. These qualitative data of the vug fraction were used further to distinguish between the different rock types.…”

Reservoir description of a mid-Cretaceous siliciclastic-carbonate ramp reservoir: Mauddud Formation in the Raudhatain and Sabiriyah fields, North Kuwait

“…The small thicknesses of such beds (often as little as 15 cm) means that, although easily identifiable in cores, in many cases they cannot be reliably detected on conventional wireline logs in uncored wells (see e.g. Akbar et al, 2000). Since the log databases for many reservoirs in the region with 15-20 years of developmental history typically contain only a small proportion of modern, high-resolution image logs, the distribution of such zones can be difficult to characterise accurately.…”

Subsurface Fracture Analysis in Carbonate Reservoirs: Kohat/Potwar Plateau, North Pakistan

“…This possibly explains why some wells in a fractured carbonate reservoir are better producers than others. With external control of log porosity and shallow resistivity, an azimuthal porosity histogram with the average and secondary porosity distribution is provided at closely spaced levels by image log analysis to address the zones and streaks of higher secondary porosity and cementation [30] and permeability [31]. [16][17].…”