Review of a Multi-Lateral Drilling and Stimulation Program
Authors: Triolo D.K. , Mathes R.A. 
Published: December 2007

Amoco Sharjah Oil Company (ASOC) is conducting a horizontal/multi-lateral drilling program in the lower Cretaceous carbonate formation of its Sajaa retrograde/condensate field which lies at a true vertical depth of approximately -12,000 ft subsea. This program has included two single laterals and three dual lateral wells (two herringbone and a stacked-style). Three wells (one single and two dual laterals) were re-entries into original vertical development wells. Production rates from these wells are three to five times the calculated rate from vertical wells. Coil tubing production logging and stimulation operations were conducted in two of the wells of measured depth &gt;15,600 ft (horizontal lateral length of 3,500 ft). Selective re-entry in the dual lateral with coil tubing was not successful. An acid wash stimulation improved the production rate by 30% in the single lateral well that was treated. The production log was capable of providing gross production rates of each lateral. A multilateral re-entry system has been installed in the current stacked-style dual lateral for future selective re-entry by coil tubing.

Introduction

Amoco Sharjah Oil Company discovered the Sajaa retrograde condensate field in December, 1980 and production began in 1982. The onshore Sajaa field is located in Sharjah, United Arab Emirates. The majority of the reserves have been developed from the Shuaiba and Kharaib carbonate formations in the lower Cretaceous Thamama group. The Shuaiba formation's top is at approximately -12,000 ft subsea and the Kharaib's top is 250 ft below the Shuaiba top. The Shuaiba is capped by the Nahr Umr shale formation.

By November 1995 the field had been developed with 28 vertical development wells. A study concluded that a horizontal well in the Shuaiba should be significantly more productive than a vertical well because of wellbore contact increase. This led to the recommendation of the Sajaa field's first horizontal well.

This paper reviews the drilling, completion and stimulation of the Sajaa horizontal wells; two single laterals and three dual laterals. The major drilling problem is geosteering in the Shuaiba formation because it has minimal logging characteristics, changing dip angles and unpredictable faults. The operations are complicated by the vertical depth of - 12,000 ft tvd and bottom hole temperatures of 295 F. The laterals are not lined, but the desire to selectively re-enter the dual laterals has led to several alterations in the completion philosophy. Completion designs have to be adapted to new wells and dual laterals which are initiated from original vertical wells. Coil tubing was used to run a production log in one of the dual laterals which allowed gross production measurement of each lateral. Selective re-entry into each lateral was not successful and the failed coil tubing acid stimulation required a work-over. The laterals were successfully re-entered with tubing and stimulated. Coil tubing was used to perform a 15% HCl acid wash of a single lateral increasing production by 30%.

Horizontal Well No. 1 - Single Lateral

The location of the first horizontal well was selected because the area had marginal productivity, but contained minimal reservoir uncertainties and dominant Shuaiba productivity. The well was drilled vertically to 10,490 ft then built to 390 at 11,478 ft (11,380 ft tvd) where the 9-5/8" casing was set in the Nahr Umr formation. An 8–1/2" pilot hole was drilled through the target formation to define its characteristics. The porosity zone identified for the lateral was between 11,975 ft and 12,020 ft md (Fig. 1). It was bounded by the Nahr Umr shale (top) and Shuaiba low porosity (bottom).

There was concern that the Nahr Umr shale below the 9-–5/8" casing would be less stable at the higher inclinations so it was protected with a 7", 35 ppf liner. P. 349^