A Case Study: Re-fracturing of High Temperature Deep Well in Carbonate Reservoir

To compare the effect of different stimulation techniques on carbonate reservoir and optimize the techniques, Well T was selected as a case study to test with Mini acid fracturing, cross-linked acid fracturing and hyrraulic fracturing in the same pay zone. The study showed that the effect of cross- linked acid fracturing was better than mini gel acid fracturing, and hydraulic fracturing technique was the best comparing with the others. The new idea and methods for the carbonate reservoir stimulation was gotten from the success of the 3 tests on the well.

Well T is a key wildcat well on some reef foreshore body. The well was an openhole well which was drilled on August 11^th, 2004 till the depth of 4925m in Ordovician. The target zone was 4913.52-4935.00m, and the reservoir temperature was 120 °C. show was good in the target zone when drilling, and the well flowed 28936m³/d gas and a little crude oil with 8.825MPa of tubing pressure using 6mm choke without any stimulation.

Mini -acid fracturing was conducted in this well and 102m³ of gel acid was pumped on September 23, 2004. The production of gas and oil were 44270m³/d and 8.26m³/d respectively using 5mm choke at 14.463MPa of tubing pressure after the mini-acid fracturing.

Surface cross-linked acid fracturing technique was conducted on the same target zone on October 17^th, 2004. 538m³ of fluid was pumped into the formation including 230m³ of pad fluid, 230m³of cross-linked acid, 56m³ of gel acid and 22m³of flush fluid. The rate was 3.5-5.6m³/min and wellhead pressure was 58-82MPa.The well produced 19906-51485m³ gas and 19.3m³ oil per day with the tubing pressure 14.99-10.03MPa using 5mm choke after the cross-linked acid fracturing.

Hydraulic fracturing was done in the same well on November 9, 2004. A total of 473.1m³ of fluid including 47m³ of proppant was pumped into the wellbore (12.2m³ 40-60 mesh and 23.3m³ 20-40 meshes). The well produced 33907m³ gas and 28.69m³ oil (crude oil density 0.8575 at 20 °C, no water) per day with the tubing pressure 9.48-10.3MPa using 6mm choke after the treatment. The result was encoursging.