Print this pageIntegrated Fracture Characterization of Najmah - Sargelu, Tight Carbonate Reservoirs, Sabriyah Structure, Kuwait: Its Implications from Hydrocarbon Exploration
Sabriyah is a NNE-SSW trending structure in North Kuwait, on the eastern flank of Kuwait Arch. The Najmah and Sargelu Formations of Middle to Upper Jurassic age have recently been established as a major gas and condensate reservoirs in Sabriyah structure. Sargelu Formation comprises of packstone facies deposited in middle ramp. The overlying Najmah Formation has two main units - lower mudstone unit deposited in outer ramp to basinal setting and upper oncoidal bioclastic packstone unit representing deposition in inner ramp setting. They are very tight with matrix porosity varying between 2% to 3% and permeability of less than 0.01md - 1.5 mD. Despite low matrix permeabilities, presence of natural open fractures in the reservoirs is believed to have enhanced the permeability by several folds, as is evident by the high production rates.
This paper aims to present the results of fracture characterization studies to provide a better understanding of the complex fractured Najmah and Sargelu reservoirs. The study was based on the integration of conventional cores and borehole image data acquired in 7 of the 9 wells drilled on the structure.Information from mud logs was included to relate mud loss zones with natural fractures identified in cores and image logs.Matrix properties play a key role on fracturing, which is best developed in the clean tight carbonates. The natural fractures in Najmah and Sargelu reservoirs were characterized in terms of size, aperture, density, orientation and sealing properties. The fractures range from 4 to 8 inches in height, however some fractures are longer and a few exceed 40 inches. The fracture aperture is 0.2 -0.4 mm. Most of the fractures are open or partially open. The fractures are steep (60 deg - 80 deg). Two dominant fracture sets were identified using borehole images, one set trends NE-SW and the other NW-SE. These trends are orthogonal to the strike-slip faults bounding the structure. High fracture density and good production potential have been observed in wells located close to the fault. Borehole breakouts suggest that the direction of principal horizontal compressive stress is NE-SW, and natural fractures that strike in this direction are mostly open. The fractures occur in clusters and these clusters are capable of transmitting fluids and enhance the permeability. Horizontal boreholes oriented perpendicular to the strike of high-permeability fracture set, hold great promise for maximizing primary production.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009