HAN BAOPING and FENG QIYAN*, China University of Mining & Technology LUO CHENGJIAN,YU JUNJI and YUN JIANGUO, Petroleum Administrative Bureau of North China

Abstract: The Study of Oilfield Karst in Buried Block Hills in Jizhong Depression

So-called oilfield karst is a kind of karst developed under oilfield conditions (temperature, PCO₂, composition and hydrodynamics of oilfield water, and features of reservoir rocks). The authors have systematically studied the the development mechaism and occurrence of oilfield karst by theoretical analysis, experimental simulation, and case histories of buried block hills in Jizhong Depression under supported by the National Natural Science Foundation of China.

Acid components are invariably present in hydrocarbon accumulations; the amount may range from several to a dozen percentages. CO₂,H₂S and aliphatic acid anions (such as CH₃ COO-) all contribute to the aggressiveness of oilfield waters. The saturation index of calcite SIc), calculation by the Oddo-Tomson equation, which considers temperature, pressure and ionic strength, has shown the most water samples in Renqiu Oilfield are unsaturated with respect to calcite (SIc from-1.51 to 0.14 and 90 percent of samples less than 0). Therefore, it bound up to corrode carbonate rocks. For buried block hills of Proterozoic-Paleozoic carbonate rocks, it has been proven that oil in their higher parts comes from the overlying Paleogene Shahejie Formation Es, Fig.1). Formation waters recharging from the overlying Es mix with paleopercolation water inside buried hills to produce mixture corrosion effects and in turn, corrode carbonate rocks to form void of oilfield karst.

In order to verify the above mentioned ideas, Renqiu Oilfield was chosen as a study area and experiments in open and closed system were employed to simulate mechanisms of karst development during exposed and buried periods for carbonate rocks respectively. Experimental rock samples are Sinion dolostone (including argillaceous, algal, siliceous and pure dolomite) and Ordovician limestone.All standard samples are pure marble, the same size as the experimental samples (4cm by 1cm by 0.5cm). Water samples for open system is carbonated water. In closed system the reaction solution is oilfield water from same boreholes where the rock samples were collected. Experimental conditions of open system are temperature 20°C and normal air pressure. According to occurrence features of carbonate reservoirs in Renqiu Oilfield, experiment in closed system employed the interaction between thermal oilfield water and carbonate rocks in autoclave at T=90°C, PCO₂=2.0MPa and with a reaction time of 9 hours. Results of the experiments are shown in Table 1, where Kv = Corrosion amount of rock sample/corrosion amount of standard marble, Kev = dissolution amount of rock sample/dissolution amount of standard marble.

Comparing the results of closed system with open system, we can find that the solubility of dolomite increases whereas that of limestone decreases relatively. It demonstrates that at great depth under higher temperature and pressure, dolomite may serve as an excellent oil or gas reservoir which is formed by the selective corrosion process of oilfield water. Reservoir of algal dolomite of Renqiu Oilfield has a maximum porosity 18.5 percent and maximum permeability 2µm².The study of case history of Renqiu Oilfield has also provided direct and convincing evidence for an origin of oilfield karst. It operates in following ways, (1) intensive corrosion of equant ferrodolomite, (2) modification of pre-existing pore, (3) new corrosion pores and caves in higher intake areas of buried hills, (4) honeycomb and slaggy structure of extrusive rock mass from oil wells and (5) partial secondary minerals precipitation (Fig.2). Reservoir properties of carbonate rocks are changed in development of oilfield karst, namely increasing heterogenetity.

Exchange action of trace elements also occurred during interaction process between oilfield water and carbonate reservoir. Reservoir dolomite in intensive zone of water-rock interaction has a lower content of trace elements such as Sr, Ba, Li, but oilfield water at the same depth has a higher content of that.

Karstific void and fissure controlled the occurrence of remaining oil.

According to its feature, some techniques have been employed to slow down the production decline in late stage of oilfield development in Jizhong Depression.They include controlling inject water rate, plugging bottom water in development well by chemical materials, using the shield action of pelitic dolomite to find remaining oil area, carrying nitrogen injection test and drilling horizontal well. These methods have led to prominent economic benefits: only Renqiu Oilfield increases 80 x 10⁴ t of oil output.

Table.1 The results of simulated experiments of typical samples

Fig.1 Geological profile of Renqiu Oilfield

Fig.2 Photomicrographs of core in well R239;A. Intensive corrosion of ferrodolomite at a depth of 3330m; B. Ferrodolomite crystals on fissure wall at a depth of 3450m; C. Needle gypsum in pore of dolordite at a depth of 3326.5m.