Achieving Maximum Horizontal Well Performance with Pre-Completion Mild Stimulation
Authors: Lynn J. D. , Nasr-El-Din H. A. , Bahamdan A. A. 
Published: December 2007

Horizontal wells have been shown to provide significant advantages for production economics. However, if damaged during drilling and completion operations, the advantage to drilling the horizontal wells is lost to high stimulation costs. The usual practice in Saudi Arabian carbonate oil producers drilled with water-based fluids (CaCO3/starch/xanthan) is to utilize well flow back for clean up. Clean-up is usually effective in wells with significant drawdown capacity, but can be incomplete in long horizontal wellbores. The drilling damage is believed to occur due to a combination of surface skin, drilling filtrate blockage, and oil-wetting of drilling solids incorporated in the filter cake. Removing this damage with mild, low-volume treatments applied during pre-completion clean-up may provide enhanced well performance at a significantly lower cost than post completion stimulation. In this "mild" application, the goal is not deep stimulation, but removal of drilling fluid generated, near-wellbore damage. The removal of skin will subsequently allow dynamic cleanup to proceed with greater effectiveness.

The options for the design of mild stimulation fluids are: 1) use low concentrations of "exotic" fluids, or 2) use cost-effective fluids with active chemicals at significantly lower concentrations. One of the "exotic" materials suggested for a mild chemical stimulation was M-S, a starch specific enzyme. In addition to the enzyme breaker, other options included various surfactant washes, and a mild acid stimulation using 3 weight percent HCl.

A preliminary evaluation of mild stimulation methods has been completed on formation cores from a Saudi Arabian carbonate reservoir, using a dynamic drilling fluid loop. The reservoir conditions laboratory core studies established that the use of an acidified brine in the pre-completion process (pH = 3-4) was not effective in removing well bore damage. The core studies further found: 1) The M-S removed approximately 50 percent of drilling fluid induced damage, 2) M-S was pH sensitive, and precipitates at values below pH =4, 3) Once precipitation occurred, the M-S was ineffective, and the precipitated solids could induce damage as a result of formation plugging. 4) A surfactant/mutual solvent wash removed approximately 65 percent of the drilling fluid induced damage, 5) The use of a mild HCl wash (3 weight percent) without a surfactant pre-wash produced a slight negative skin, and 6) The use of the surfactant pre-wash, followed by a mild HCl acid treatment produced the largest negative skin. The core faces of the acid washed samples did not show wormholes after circulation of the mild acid treatments.

M-S was found to reduce the molecular weight of starch used for drilling applications, but had no effect on the xanthan polymer used in drilling fluids.

The data from the core study were used to design a field application for a Saudi Arabian horizontal producer that had been damaged during drilling. Initial results indicated that formation damage due to drill-in fluid was removed after M-S treatment followed by an acid wash. Additional test wells are planned to investigate other stimulation options.