Summary: We have investigated the seismic velocities, their variations and dependencies, in wells that penetrate the limestone reservoirs from Kashagan oil field. Porosity alone controls the seismic-wave velocities of the intact limestones. Fluid saturation has little effect on intact rock because the rock frame is so stiff. A high density of fractures decreases the frame stiffnesses. A high density of fractures decreases the compressional and shear wave velocities, correspondingly. In highly fractured zones, bitumen and water increase the compressional wave velocity significantly relative to oil and gas.
These results were derived from testing simple conceptual models against log data. The models were developed to describe (a) the intact limestone frame in low-porosity limestone, (b) fluid saturation including gas, oil, water, and bitumen, and (c) fracturing. A key concept is that the highly fractured zones consist of completely fragmented rock as opposed to slightly cracked rock.
Introduction: We investigate the effect of fracturing on the seismic velocities, Vp and Vs, in low-porosity limestones from the Kashagan oil field in Kazakhstan.
The Kashagan field is located in the north-eastern part of the Caspian sea about seventy km south of Atyrau city, see Figure 1. It is covered by shallow water with an average water depth of 4m. The reservoir is a large isolated carbonate platform which accumulated in a shallow marine environment. With the closure of the Caspian Basin a huge Permian evaporitic sequence was deposited. Later Permo- Triassic and Jurasic-Creatceous clastic deposition was accompanied by strong salt diapirism that created complex salt domes and Mesozoic minibasins. The field is now moving toward the production phase. In order to optimize the planning of the future drilling activity, it is necessary to better understand the fracture network that is difficult to see using conventional 3D surface seismic data.
Laboratory experiments have shown that sonic velocity of carbonates is mainly controlled by porosity and pore types (Wang, 1997). The effect of saturation on velocity in carbonate rocks, nevertheless, has been evaluated in few studies. The selection of a proper rock physics model to estimate velocities is still not well known therefore particular attention must be done when applying rock physics on carbonate rocks.
We proceeded with a conceptual study that relied on testing with well logs and presumed simple behavior for fractures. The modeling presumes a rock frame composed entirely of pure limestone, stiffnesses in parallel between the rock frame and the pore space, and a granular, fully-fragmented limestone in series with an intact, porous limestone. It depends on endpoint properties.
Approach: Treat intact and fully fragmented limestone as distinct end-member rock-fabric elements,
develop a model for intact limestone,
develop a model for fully fractured and fragmented limestone,
combine the intact- and fragmented-limestone rock-fabric elements using a springs-in-series approach and a simple linear scaling factor, and
compare the heuristic model to well-log data.
We have developed a heuristic model for fractured limestones. Our approach was to: