Evaluation and optimization of water-salt based drilling fluids for slim-hole wells in one of Iranian central oil fields

Slim-hole drilling in oil and gas industry has been developed extensively in recent years. Drilling fluid, in addition to providing appropriate rheological properties should produce a low annular pressure loss (APL) gradient while drilling. During slim-hole drilling, drilling mud hydraulics is an important concern, because due to reduced annular clearance, pressure loss can occur in drill pipe and annulus considerably. In this research, water based drilling muds were selected for the experimental work due to its low price, simple preparation and easy access to the required water. The research includes development and testing of water-salt based fluids. In this study, sixty-five samples of fluids were analyzed for their rheological parameters, the were considered using three various case study gas field wellbore configurations 6 1/8, 5 7/8 and 4 1/8 inch for their calculate APL gradients. Each composition was evaluated by the Power-Law and Bingham Plastic models and results of both models were compared. Choose of optimum fluids is based on suitable rheological properties, minimum annular pressure losses and maximum fluid transport ratio. Also, the effect of high temperature (180°F) on frictional pressure losses was studied. For an optimal formulation at high temperature from a well with distinct configuration 6 1/8 inch had an APL gradient of 0.149 psi/ft compared to 0.176 psi/ft at atmospheric condition. It was found that effect of high temperature on drilling fluids behavior is affirmative. On the other hand, the observed effect of high temperature on Power-Law fluids greater than Bingham Plastic fluids. Fluids with xanthan, in spite of high cutting carrying capacity and due to high viscosity have exhibited high frictional pressure loss. However, for some fluids, the annular pressure loss increased at high temperature due to green starch fermentation.