A new technique to improve primary cement jobs has resulted in a 35% decrease in average remedial cementing costs for a group of Canadian gas wells.

Production optimization begins with a good completion, and a good completion is based on the integrity of the primary cement job. Unfortunately, about 15% of all primary cement jobs fail, costing the oil and gas industry an estimated US $470 million annually.1 One-third of these failures is attributed to fluid influx into the cement-filled annulus.

According to David Stein of Reservoir Isolation Technology Services (RITS), many of these failures could be avoided by employing a simple procedure known as cement pulsation (CP). CP is the application of low-intensity pressure pulses to the annulus to delay gel strength development in the cement slurry. Developing gel strength in the setting cement causes a lowering of the hydrostatic pressure transmitted through the annulus.1 When the pressure differentials are small, this can lead to fluid influx from permeable formations. By delaying gel strength development, the hydrostatic pressure on the formation is maintained until the cement has built sufficient strength to prevent the influx and migration of reservoir fluids through the cement matrix. "The pulsation process starts immediately after pumping stops and the annular BOP is closed," said Stein. "We apply low-pressure pulses to the casing annulus, about 80- to 200-psi pulses at 30- to 60-second intervals, using water pressurized by an air compressor." Pulsing continues until the volume of fluid required levels off, usually 4 to 6 hours.

Gas Technology Institute (GTI) and Texaco E&P Technology Co. collaborated in the early development of this simple procedure during the 1990s. More recently, CTES LC further developed the system and collected downhole annulus pressure data during initial testing. After verifying that the pressure pulse did in fact transmit completely through the cement column, a field trial was conducted on a group of Canadian wells.2

CP was applied during 2000-2002 on more than 150 wells in areas of Alberta and Saskatchewan that historically have had problems with gas migration. Typically between 1,900 ft and 5,900 ft (580 m and 1,800 m) deep, the wells in these areas developed leaks after primary cementing in 57% of the cases, on average. Repairing the leaks as required by Canadian regulations could cost anywhere from $30,000 to $50,000, according to Stein.

"Comparing the expected number of leaking wells based on historical behavior in each of the top 11 fields with the actual number of leaking wells after pulsing, we saw that overall 57% average drop to 21% (see figure). The overall average cementing cost for primary-plus-remedial cementing dropped by 35%," said Stein.

While cement pulsation is not a solution for all primary cementing problems in every well situation, it could provide a relatively inexpensive insurance policy (estimated at $5,000 to $9,000 depending on the logistics) for wells with a delicate margin between fracturing pressure and pore pressure. According to Stein, work in Canada is still continuing, and RITS is gearing up to make the service available in the United States. For more information, contact Stein at david.stein@rits.cc, or visit www.rits.cc.

References
1. Newman, K.; Wojtanowicz, A.; and Gahan, B. "Improving Gas Well Cement Jobs with Cement Pulsation" GasTIPS, pp. 29-33, Fall 2001.
2. Dusterhoft, D.; Wilson, G.; and Newman, K. "Field Study on the Use of Cement Pulsation to Control Gas Migration," SPE paper 75689, 2002.



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