Liner Top Milling Technique Integral to Successful Drilling of Uwainat Horizontal Sidetracks in Offshore Qatar
Authors: Simpson M.A. , Latif S. 
Published: December 2007

Abstract
This paper describes the production liner milling and sidetracking techniques used by Qatar General Petroleum Corporation (QGPC) for Uwainat Formation Horizontal Sidetracks. The Uwainat Reservoir is a 200 ft thick Jurassic Carbonate at 9000 ft TVD present in Maydan Mahzam(MM) and Bul Hanine(BH) fields, 70 nautical miles offshore Qatar in the Arabian Gulf.

Introduction
With the success of the first Uwainat Horizontal well, several existing BH and MM Vertical Producers were targeted for Horizontal Side-tracks into the reservoir. Most of the existing Uwainat wells had 7" production liners set across the Uwainat Reservoir, with 300 ft overlap into the 9 5/8" intermediate casing set in the massive Arab "D" formation at 7700 ft TVD (Fig. 1). Section milling a window in the 9 5/8" or 7" casing would have resulted in a 6 1/8" horizontal section across the Uwainat, whilst reservoir and drilling engineering preferred 8 1/2" hole. However, if 426 ft of 7", 29 ppf, L-80 Buttress casing liner could be milled to 100 ft below the 9 5/8" intermediate shoe, the entire horizontal section could be kicked off and drilled in 8 1/2" hole in well BH-29A.
The first horizontal side-track well, BH-29A was subsequently drilled by first milling out 426 ft of 7" liner to gain access to the 8 1/2" open hole. A 10.0 ppg Xanthan Gum/Bentonite based milling fluid with minimum, 45, 6 rpm viscometer reading was utilized in conjunction with 8 1/2" junk mills to mill out the 7" Liner PBR, liner top packer and hanger assembly at an average rate of 1 fph. The 8 1/2" section mills × 6" Taper mill assemblies were used with milling rates of up to 31 fph on the 7" liner body. An average overall rate of 26 fph for pipe body and couplings was achieved. The milling job was completed in 5 days with a total cost of $239,000. The well was then side-tracked in 8 1/2" hole and the medium radius 4,026 ft horizontal section drilled in the Upper Uwainat, incorporating 1.75 deg/100 ft of left-hand turn. The well was Production tested on a 1" choke and produced 3900 BOPD as compared to 300 BOPD in the original wellbore before the side-track. The entire cost of side-tracking BH-29A including milling, drilling and logging was $1.23 million, about one half the cost of drilling a new horizontal well.
Since BH-29A, 10 additional Horizontal Side-tracks have been drilled in the Uwainat, Arab "C" and "D" reservoirs by QGPC. Wells MM-4B, an Arab "A" Side-track, Deviated Arab "B" Horizontal Dual Completion and BH-23 A & B, an Arab "C" Sidetrack, Multilateral Re-Entry Completion are currently being drilled. This paper details the Completion Retrieval and Abandonment Techniques, Milling Equipment, Milling Fluids, Surface Swarf Handling Equipment, Operating Parameters and Side-Tracking Techniques successfully employed on the QGPC Side-tracks.

Completion Retrieval and Abandonment
The 3-1/2" Completions were retrieved after squeeze killing the reservoir with 9.9 ppg brine in the case of the Uwainat wells or 8.6 ppg brine or sea water for the lower pressured Arab "D" wells. Losses to the formation were typically cured with viscous pills and medium to fine Calcium Carbonate. The permanent packer was milled and retrieved with a packer-retrieval tool or pushed to bottom if unretrievable. The existing perforations were then low pressure, hesitation squeezed, by the Braden head method or with a down hole squeeze tool. A drillable bridge plug was set above the abandoned interval with a column of cement spotted above it. All abandonments and testing of plugs were done in accordance with U.S. Minerals Management Service Regulations.
On the most recent horizontal sidetrack, MM-38, the reservoir was cement squeezed with 1-1/2" coil tubing through the completion. P. 161^