Analysis of Reservoir Protection Mechanism by Direct Drilling Fluid Flowback

Horizontal well open hole completion is a way of well completion to maximize well production rate. In offshore drilling, well completion is generally done with solids-free drill-in fluid, supplemented with gel-breaking well completion. Direct flowback of drill-in fluid is a technology developed on the basis of this way of well completion. In this paper, parameter design, additive selection and well completion technique were systematically analyzed and evaluated, and the mechanism of reservoir protection of this technology was also analyzed. The composition of the drill-in fluid is as follows:seawater+soda ash/caustic soda+rheology modifier VIS+starch filter loss reducer STARFLO+soluble salt (shale inhibition and mud weight)+high purity sized calcium carbonate MBA. The three functional additives of the formulation can all be liquefied by 0.3% HTA (a chelating agent) solution. The liquefaction of the additives left no residue and the mud cakes in the wellbore and near the borehole wall were all converted to clear saltwater. MBA is a mixture of calcium carbonate powders with five particle sizes. The particles of MBA can be "inter-inserted" with starch to form a mud cake that is much denser. Laboratory experiment has shown that the drill-in fluid had low-shear-rate viscosity of around 30,000 mPa·s, optimum contamination-resistance and flowback ability, and the highest permeability return. After contaminated with 15% drill cuttings, the drill-in fluid still had permeability return of more than 80%. This drill-in fluid was used in twelve wells in three oil fields and one gas field in the east of South China Sea. The maximum temperature at the hole bottom was 60-130℃, and the highest mud density was 1.20 g/cm³. Three of the twelve wells penetrated low porosity low permeability reservoirs and the rest nine wells penetrated reservoirs with medium to high porosity and permeability. The drilling operation was successful. The use of this technology simplified the process of well completion, saved operational time and cost, and protected the reservoirs from being damaged.