Delayed-Release Acid System for Cleanup of Al Khalij Horizontal Openhole Completions
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98164, "Delayed-Release Acid System for Cleanup of Al Khalij Horizontal Openhole Drains," by P. Leschi, SPE, and G. Demarthon, Total E&P, and E. Davidson, SPE, and D. Clinch, Halliburton, prepared for the 2006 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 15-17 February.
The full-length paper describes a new approach that uses a slow-acting stimulation treatment for damage removal and stimulation. The stimulation treatment comprises a starch enzyme to degrade the most troublesome polymer and an organic compound that reacts with the carrier brine to release organic acid in situ over a period of several hours. The system has been used recently in the Al Khalij field in Qatar with openhole lengths ranging from 162 to 576 ft and treatment volumes ranging from 125 to 725 bbl.

Introduction
The oil-bearing Mishrif reservoir is a stratigraphic trap in the Upper Cenomanian limestone. Laid down with several repeating phases of deposition and erosion, the reservoir typically consists of 20- to 30-ft sequences, each alternating between poor-permeability matrix intervals and highly productive diagenetically weathered layers. Permeability estimated from test data ranges from 100 md to a few darcies in drain layers and 1 to 50 md in matrix intervals.
The oil-bearing strata are at approximately 366 ft true vertical depth, and the Mishrif formation can be as much as 46 ft thick. The formation is normally pressured, and bottomhole temperature (BHT) is 127°F.
During initial development, it was thought that vertical or slightly deviated wells with electrical submersible pumps (ESPs) would be used because the naturally high permeability could be expected to provide good productivity. However, there was a change in plan when permeability was found to be very irregular. This prompted the decision to develop the field with horizontal wells with long horizontal sections. Subsequent development work on this field concentrated on optimizing the drilling process and improving acidizing treatments for cased-hole wells. Great success was achieved by the combined use of retarded and emulsified hydrochloric acid (HCl) and ball sealers for mechanical diversion.

Cased or Openhole Completions
In general, fractured reservoirs offer the best prospects for openhole completions, but only if there is no need for selectivity and if wellbore damage can be cleaned up satisfactorily. Openhole completions offer several benefits including cost effectiveness and ease of execution. In limestone completions, some openhole-wellbore stimulation usually is necessary to remove drilling damage.
However, in some formations, deep acid penetration is required. In such cases, acid must be forced to penetrate several feet into the rock, either by creating large wormholes or by fracturing. Such operations require a cased-hole completion and use of mechanical and chemical diversion techniques. The major disadvantage of these completions is that the extra materials and time required for the casing, cementing, and perforating operations result in significantly higher costs.