Study On Water Infusion Damage Mechanism Of Cave - Type Carbonate Reservoir

Carbonate oil and gas resources which mainly occurs in the Tarim Basin, Sichuan Basin, Ordos Basin, Bohai Bay Basin are rich in China. Because of weak water body energy, the annual decline rate of single well and multi-well fracture-vuggy units are respectively from 30% to 90% and from 20% to 40%.Single well water injection substituting oil and unit water injection pilot study which creatively carry out by Tahe oilfield succeed in raising the productivity. But the added new water injection difficulties wells are the proportion accounted for 26.7% of the total added new water injection wells. Therefore, it has important practical significance to understand the water injection damage mechanism.Taking Ordovician carbonate reservoir of Tahe Oilfield 12 area as the research object, the engineering geological characteristics of the study area and loss impacting on water injection operation were investigated, the potential reservoir damage factors was revealed, rate sensitivity, water sensitivity, salt sensitivity and alkali sensitivity, lost solids and injected water suspended solids invasion damage experiment, and solids invasion in to fracture core stress sensitivity test were conducted, water injection scaling tendency was forecasted, water injection corrosion factor were analyzed, fracture-vuggy formation water injection damage mechanism were analyzed, control measures of water injection damage mechanism were probed.The results show that fracture-vuggy carbonate reservoir have complex space system and pores, fractures, cavities developing. During drilling and completion, drilling and completion fluid loss is severe, lost solids invaded into fracture-vuggy system, the object of water injection from original formation change to formation which is existing lost solids.Under water injection pressure differential effect, lost solids along with the injected water invaded into deep formation. Leakage Solids mix with injected water suspended solids changing the size distribution and concentration distribution of injected water.Survey region are rich in fracture and sensitive mineral. Experiments evaluate the fluid sensitivity of fracture sample. The results show that the extent of rate sensitivity, salt sensitivity and alkali sensitivity are respectively for medium to strong, weak to medium, weak and strong.The article using SI, SAI and the thermodynamic solubility method separately predict the tendency of calcium carbonate scale and calcium sulfate scale in injected water. The results show that injected water temperature of Tahe oilfield from 40℃ to 80℃ will produce calcium carbonate scale, and will not produce calcium sulfate scale.The concentration of Cl ion in injected water of work aera is 130427 and the pH is 6.1,they can cause electrochemical corrosion and corrosion products were carried into the reservoir fluid.For vuggy units, the invasive solids can not enough to form jam, but we should consider the effect of solids deposition and jam on the network of fracture which is in connection with vuggys. By means of experimentation, researching leakage of solids and injected water suspended solids invaded in to fracture sample of the extent of the damage. The damage rate of permeability of leakage of solids invasion fracture sample are between 79.10% and 93.10%, and the damage rate of permeability of suspended solids invasion fracture sample are between 83.70% and 90.41%. By means of contrast injected water suspended solids and mixed solids which are suspended solids and leakage of solids invaded into fracture-vuggy by stochastic simulation method. The results show that the extent of damage making by mixed solids are more seriously than suspended solids. Solids invasion affect the fracture closure behavior under stress, and the stress sensitivity is obviously strengthened. The combined effect of the solids invasion and stress sensitivity making fracture width reduced from 1/10 to 1/5 of the initial width, resulting particle size of injected water suspended solids reduced from 1/10 to 1/5 of the initial size.Because of leakage solids will increase injected water suspended solids particle size and concentration distribution, and strengthen the formation stress sensitivity, and lessen seepage channel width, therefore, it is important to strictly control the injected water suspended solids particle size index, at same time, it is necessary to control drilling and completion fluids loss. Suggested that in the drilling process, The use of shielding temporary plugging or temporary plugging technology control fractured loss.