TABLE OF CONTENTS Fundamentals of fluid flow in gas, stable foam, and aerated liquid drilling systems are presented in chapters 2-5. Simple application calculations are also demonstrated in those chapters. This chapter demonstrates how to develop the optimum designs of gas and liquid flow rates for underbalanced drilling projects. A liquid - gas rate window approach is presented, and a field case study is demonstrated.
For a given hole geometry (hole and pipe sizes and depth) and fluid properties, the surface choke pressure, liquid flow rate, and gas injection rate are three major parameters that affect bottom-hole pressures during drilling and circulation-breaking conditions. The liquid flow rate and gas injection rate should be carefully designed to ensure underbalanced drilling and wellbore integrity. The liquid - gas flow rate window (LGRW) described in this section defines the margins of useable liquid and gas flow rates in underbalanced drilling.
The combination of liquid flow rate and gas injection rate should be carefully designed so the flowing bottom-hole pressure is less than the formation pore pressure under drilling conditions and the circulation-break bottom-hole pressure is greater than the formation collapse pressure. Other considerations in designing liquid and gas flow rates include cuttings-carrying capacity of the fluid mixture and wellbore washout. The former defines the lower boundary of useable flow rate combinations, and the latter defines the upper limit.
A typical LGRW is illustrated in Figure 6-1. It can be prepared using various...
