A Study of Solid-Free Drilling Fluid for Tight Gas Reservoirs

This project is explaining a laboratory development of a solid free drilling fluid formula that could be potentially used in tight gas reservoirs. The configuration of the weak gel fluid WGL-1, which is resistant to high temperature and high salt, was tested, and concluded that its gelling properties, salt and temperature resistance, and environmental protection were all in line with industry requirements. The final drilling fluid formula was developed as: water + (0.3% ~ 0.5%) NaOH + 5% KCl + 2% WGL-1 + 5% NaCl + (1.0% ~ 2.0%) HBFR Anti-high temperature fluid loss agent + 2% Polyol + (1.5% ~ 2.0%) SDL-1 Lubricant + 0.4% A4O1. The performance of the liquid was tested for temperature resistance, inhibition, gas formation protection effect, plugging performance, and static settlement stability. It was concluded that the temperature resistance performance is satisfied at 150°C, and the cuttings recovery rate is as high as 96.78%. It has good performance in inhibiting water dispersion and swelling of cuttings. The permeability recovery value reaches 88.9%, which meets the requirements of gas formation protection. The SSSI value shows that its settlement stability is good;under high temperature and high pressure, its sealing performance is good. This drilling fluid system has achieved the expected results and laid a foundation for further promoting the development of solid-free drilling fluid systems. The future development direction of solid-free drilling fluids is pointed out, to the improvement of properties to be applied in high temperature environment and have high salt resistance capacity.

Designing Drill-in Fluids by Using Ideal Packing Technique

Selecting bridging agents properly is a critical factor in designing non-damaging or low-damaging drill-in fluids. Historically, Abrams＇ rule has been used for this purpose. However, Abrams＇ rule only addresses the size of particle required to initiate a bridge. The rule does not give an optimum size nor an ideal packing sequence for minimizing fluid invasion and optimizing sealing. This paper elaborates an ideal packing approach to solving the sealing problem by sealing pores with different sizes, especially those large pores which usually make dominant contribution to permeability and thereby effectively preventing the solids and filtrate of drill-in fluids from invading into formations, compared with the conventionally used techniques. Practical software has been developed to optimize the blending proportion of several bridging agents, so as to achieve ideal packing effectiveness. The method and its use in selecting the best blending proportion of several bridging agents are also discussed in this paper. A carefully designed drill-in fluid by using the ideal packing technique （named the IPT fluid） for offshore drilling operations at the Weizhou Oilfield, Nanhai West Company, CNOOC is presented. The near 100% return permeabilities from the dynamic damage tests using reservoir cores demonstrated the excellent bridging effect provided by this drill-in fluid.