Abstract

It is well known that the use of hydrochloric acid to clean up and restore permeability of open holes drilled in limestone formations is a questionable procedure. It is necessary to remove, not only the filter cake at the well bore face but, more importantly, the low permeability crushed zone created during the drilling operation. To achieve uniform treatment of the entire openhole section with hydrochloric acid is difficult: the rapid reaction of the acid in downhole conditions often creates a localized loss zone, through which most of the treating fluid is lost so that treatment of the entire section is inefficient.

Traditional completion practice on Al Khalij field (Qatar) involved cemented casing, perforations and subsequent stimulation of the limestone with retarded emulsified hydrochloric acid and ball sealers. This paper describes a new and different approach, which involves leaving drains in openhole condition and using 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 breaker fluid is introduced to the openhole section in a neutral pH condition, thus enabling it to be distributed over the entire interval of interest. Enough acid is generated over the subsequent 12 hours to remove the filter cake and clean up the crushed zone.

This system has recently been used on several occasions on this field, with openhole drain length ranging from 530 to 1890 m (1,740 to 6,200 ft) and treatment volumes ranging from 20 to 115 m3 (125 to 725 bbl). Losses from these wells occurred after the predicted elapsed time. Details are provided of how the jobs were carried out and the results achieved.

Introduction

The Al Khalij offshore oilfield is in the northern part of the Qatar sector of the Arabian Gulf.

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 sequences (20 to 30 ft), each alternating between poor permeability matrix intervals and highly productive diagenetically weathered layers (drain layers)[1]. Permeability estimated from test data ranges between 100 mD to a few Darcy in drain layers but, in contrast, matrix permeability is 1 - 50 mD.

The oil bearing strata are shallow at about 1200 m TVD. Total thickness of the Mishrif formation can be as much as 150 m (∼500 ft). The formation is normally pressured and bottomhole temperature is low at about 53°C.

During the initial development stage of this field, it was considered that vertical or slightly deviated wells, activated by means of ESP (Electrical Submersible Pump), would be appropriate as it appeared that the natural high permeability drains could be expected to provide good productivity[2]. However, a change in plan was brought about when the drain permeability was found to be very irregular. Good permeability was found to occur in random zones of around 200 - 500 m and this prompted the decision to develop the field using horizontal wells[2], the intention being to drill very long horizontal sections to intersect the productive zones.

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