Genesis And Characterization Techniques Of Carbonate Reservoir Under Overpressure

In this paper, a case study is made of the abnormally high-pressured pre-saltcarboniferous oil reservoir of Kenkyak lying in the eastern margin of the Caspian Seabasin. By the aid of core data, well logging data, well test results, seismic attributeanalysis and seismic inversion data, etc, an in-depth research has been done in thegenesis, classification and characterization of abnormally high-pressured carbonatereservoirs, an identification technique of carbonate reservoir types is invented, and acarbonate reservoir classification template is established. On the basis of researches inthe prediction of lithofacies, karst, fractures and reservoir properties, a threedimensional dual medium geological model of the reservoir is built, and threedimensional visualization of the reservoir is realized.Under the salt formation sealed condition, the integrated influence ofhydrocarbon cracking, tectonic movement, diagenesis and hydrocarbon accumulationgave rise to the abnormally high pressure of the pre-salt carbonate oil reservoir ofKenkyak, resulting in hydrofractures. The simulation results of ground stress fieldindicates that the maximum pore pressure in Kenkyak oil field in geological historyreached118MPa ever. Meanwhile, the experiment of acoustic emission reveals thatthe minimum horizontal principal stress is86MPa, the maximum tensile strength ofthe reservoir rock being9.1MPam, which meets the key requirement for theformation of tension fissures, that is, the pore pressure is greater than the sum oftensile strength and the minimum principal stress (say118is greater than the sum of86and9.1here). As a result, the reservoir rock cracked to form hydrofrcatures inKenkyak oil reservoir, which dominantly comprise of fissures including newlydeveloped ones and re-opening, stretch and extension of old fractures.The pores of pre-salt carboniferous carbonate oil reservoir of Kenkyak are ofvarious genesis, different sizes, and the reservoir connectivity is principally controlledby its physical properties. Both fractures and fissures of different scales and genesisdevelop and are widespread, weaving a complicated fracture system. The relationshipbetween permeability and porosity is complex with reservoirs of different type havingdifferent relationship equations, and the effective permeability calculated from welltesting data is as many as ten times of that from core data. Based on the recognitionthat the assemblage of pore system and fracture system is characterized by differentseepage features and regularities, the carbonate reservoirs of pre-salt Kenkyak oilfield are classified into four categories, that is, reservoir of pore, reservoir ofpore-fracture, reservoir of fracture-pore and reservoir of fracture. With the integratedutilization of static data and dynamic data, and with the understanding to pore textureand their seepage features, the quantitative identification criteria of reservoir type areestablished, and the classification template of dual medium carbonate reservoirs aremade. According to the spatial distribution and assemblage regularity of reservoirs of different types, it is concluded that the high permeability area is characterized bymassive reservoir, and the low permeability area by laminated reservoir, which differsfrom the former recognition that the whole field is of laminated characteristic.The favorable Carboniferous carbonate reservoirs of Kenkyak oil field mainlylocate in platform rim and eroded slope zone where solution pores and fractures arewell developed. Based on the prediction of reservoir properties and fractures, takingwell logging data and seismic attributes as constrains, facies-controlled andproperty-constrained three dimensional geological model of matrix are built.Furthermore, on the basis of core data and image logging data, taking into account ofthe ant-tracking and coherent analysis results, the fracture network model includinglarge-scaled and small-scaled fractures are established by fracture groups, based onwhich, fracture flow units are constructed. Consequently, it is determined that thedrainage radius for wells in high-permeability area is larger than800meters, and thatfor wells in low-permeability area is about250m.Based on the researches in genesis, classification, prediction and characterizationof the abnormally high-pressured carbonate reservoirs of Kenkyak oil field, newrecognition are gained, and a reliable geological model is built, providing a soundinggeological foundation for the further development of the oil field.