Abstract:

The fractured-vuggy carbonate reservoirs with low porosity and low permeability basically have no oil and gas storage capacity. Fractures and caves are the main storage and permeability spaces, but fractures and caves have complex developed space and poor connectivity. Due to the limited sizes of fractures and caves during drilling in single well, and the failure of conventional water injection and acid fracturing reforms to achieve efficient expansion in remote well areas, single well production has high initial production, rapid decline, and low production. Therefore, a field test of the high-pressure water injection volume expansion process was carried out, that is, using high-pressure water injection as an energy carrier to break through the barrier of an isolated fractured-vuggy system and achieve effective expansion. This process was tested in the well-S1 of Tahe Oilfield, and the effect was significant. The well was subjected to conventional water injection and oil replacement for 3 rounds in the early stage, and the effect gradually became worse. The production is characterized by constant volume. The seismic data show large-scale of the fractured-vuggy system around the well. Two rounds of high-pressure water expansion test are carried out, with the cumulative water injection of 2.98×10 ⁴ m ³ and maximum injection pressure of 23 MPa. The water injection curves show that the high-pressure water injection make two fractured-vuggy systems around one well communicate with each other, improving the production status of this well. The cycle water content is reduced from 45 % to 4 %, and the production pressure drop is reduced from 29 MPa to 26 MPa. The production volume per unit pressure drop increased from 201 m ³/MPa to 547 m ³/MPa. So far, the cumulative oil increase has been 14 721 t. The tests show that the high-pressure water injection expansion technology can achieve the effective expansion of fractured-cavity carbonate wells with constant-volume, increase single-well production and reserves, and at the same time, create space for reducing the oil-water interface near the well, so as to extend the dry oil recovery period.

Key words: carbonate reservoir, far-well fractured-vuggy systems, water injection for displacement of oil, high pressure water injection expansion, water injection for displacement of oil