Ron Crook and James Heathman, Halliburton Energy Services Inc GAS MIGRATION CREATES permanent channels in ce- ment columns, decreasing cement strength and contributing to continued gas-flow problems. However, operators can accu- rately predict the potential of their wells to be troubled by gas flow. Based on the severity of the gas flow problems expected, the operator can avoid remedial squeeze jobs by determining the most effective cementing strategy for the situation. “Annular gas flow”, “gas migration”, and “gas leakage” are all terms that refer to formation gas that enters a cemented cas- ing/borehole annulus, creating permanent channels and weak- ening cement compressive strength. There are two major types of gas migration: short-term and long-term. Short-term gas mi- gration occurs before the cement sets, and long-term gas mi- gration develops after the cement has set. Sutton, Sabins and Faul 1,2 published definitive work in 1984 presenting (1) annular gas-flow theory and evaluation for annular gas-flow potential, and (2) tracing the evolution of gas-flow theory and preventive practices. CAUSES OF SHORT-TERM MIGRATION The most widely accepted cause of short-term gas migration is the cement column’s inability to maintain overbalance pres- sure. This pressure loss depends on 3 factors: the cement’s de- velopment of static gel strength (SGS), transition time, and hy- dration volume reduction. Static Gel Strength. In a true fluid system, hydrostatic pressure is present. After the cement slurry is placed downhole, it initial- ly acts as a fluid and exerts hydrostatic pressure on the gas- bearing formation. This overbalance pressure helps prevent 40 D R I L L I N G C O N T R A C T O R November/December 1998 Predicting potential gas-flow rates to help determine the best cementing practices A B C D Filtrate Loss Filtrate Loss Filtrate Loss Gas Channel Permeable Zone Gas Zone Figure 1: Gas channel migration: Gas migration creates permanent channels in cement columns, decreasing cement strength and contributing to continu- ing gas-flow problems. The most widely accepted explanation for short-term migration is the cement column’s inability to maintain overbalance pressure. This pressure loss depnds on the cement’s development of static gel stength, transition time and hydration volume reduction. There are 2 suspected caus- es of long-term migration: inadequate drilling fluid displacement and cement debonding. Inadequately displaced drilling fluid can prevent good bond for- mation between the pipe and cement and/or the cement and the formation. Incomplete displacement or excessive filter-cake buildup can create drilling fluid channels in the cement. Over time, gas flow causes the drilling fluid and cake to dehydrate and shrink, resulting in a highly permeable pathway for gas.