Research Of Water Plugging Mechanism For Carbonate Reservoir In Tahe Oilfield

The fracture-cavity carbonate reservoir of Ordovician formation in Tahe Oilfield has complex feature which includes tight matrix, rich fracture and cavity that are diversity in size or shape and uneven distribution, abundant bottom water.The rapid increase of water cut play one of the main role during the development which result in fast natural decline and significant decline in production, so, the key for improving overall development result and enhancing the oil recovery in oilfield is to slow down the velocity of water cut rise after the reservoir began to produce water. To achieve such goal, some water plugging technologies have been developed although there are some difficulties which must be faced including deeply buried(greater than 5400m ) and high temperature(greater than 125℃) reservoir, high salinity (19-23×104mg/l) formation water and high crude oil viscosity(46mPa·s in the area6 of Tahe oilfield) at reservoir conditions.Based on the chemical water plugging project implemented in 24 wells from 2001 to 2005, characteristic of water flooding in oil well, the law of oil-water flow, the effect analysis of water shut off, experiences and lessons learned from the field experiment, the core physical model used for evaluating the effect of water shut-off in laboratory has been set up in this paper. After a large number of tests in laboratory, the plugging agent system suitable for high temperature and high salinity fracture-cavity carbonate reservoir and the technology of selective blocking water channel in heavy oil reservoir have been chosen and determined.As a result, the following main results and recognition has been achieved.Firstly, for fractured reservoir, because of both the strongly shielding effects of wide fracture over narrow fracture during the oil-water mitigation process and the mobility difference between the oil and water, the three slug water shutoff process containing front temporary blocking agent should be applied for the poor effect of water shut-off by using conventional program. Also, because even in high water cut phase in production well, the water may penetrate via the narrow fracture other than the wide fracture.Secondly, the formula containing temporary blocking agent and the formula containing modified tannin blocking agent and flexible blocking agent suitable for the reservoir with high temperature, high salinity and water chancel have been screened respectively. After 40-60 minutes, the gel with apparent viscosity 20,000mPa·s at formation temperature from formula containing temporary blocking agent can be formed and the gel will automatic break in 15hrs. The gel from formula containing modified tannin blocking agent has high temperature and high salinity stability. The gel from formula containing flexible blocking agent is also long-term stable and easy to break down.Thirdly, the dynamic simulation experiments based on the core physical model used for evaluating the effect of water shut-off in laboratory shows that when the fracture producing water is smaller than the fracture producing oil, the temporary blocking agent has preferential access to the oil channel and form high-strength gel barrier in oil channel. So, the main blocking agent following the temporary agent has access to the water chancel and the goal of selective water shutoff can be achieved. The gel from formula containing modified tannin blocking agent has more shut off capacity in water level because of the uniform gel blocking barrier easily combined with the water-bearing rock surface and selective water shutoff ability in oil level because of the formed gel easy breakthrough and channel generation due to the coexistence of residual oil and the gel. At the early injection stage of blocking agent, the major effects exist in the increasing thickness of seal level and significant impact on breakthrough pressure gradient. When the thickness of blocking level reach a certain degree, the main effects reflect in narrow flow chancel and distinguished influence on the residual resistance coefficient because of the accumulation of blocking agent.