Various Horizontal Technologies Used to Develop Shallow Gas Sands in the Eugene Island Area
Authors: Schroeder Tom , Mathis D.E. , Mathis Anne , Hill W.A. , Ellis Andy , Hea Robert G. 
Published: December 2007

In early 1996, Vastar Resources completed a three-well project in the Eugene Island area. These wells were drilled using various horizontal technologies to develop three thin (10-22 ft) shallow gas sands. (Target TVD for one well was only 1,060 ft.) One was an "extended reach" well with a vertical section of 7,450 ft. Economics of the project were optimized by "batch drilling", that is, drilling to target depth on all wells prior to any horizontal drilling. Accurate geosteering was accomplished using near-bit MWD sensors integrated with a directional drilling motor. Before drilling, MWD resistivity models were created; these models were compared with real-time data to help identify proximity to the edges of the targets during horizontal drilling. To minimize risk, the wells were drilled to targets defined by offset well control, pilot holes, and seismic amplitude anomalies.

Introduction

The Eugene Island project area is south of St. Mary Parish, Louisiana, in the Gulf of Mexico (Fig. 1). Productive sands occur from depths of 930 ft (Holocene) to 5,600 ft TVD subsea (Middle Pleistocene). Cumulative field production is 560 BCFG and 166 MBC. Target sands for horizontal development are shown in the type log in Fig. 2 - the A (1,060 ft TVD), the B (3,070 ft TVD), and the C sand (4,145 ft TVD). (Note: TVD values are referenced to the planned horizontal wells and therefore differ from the type log depths.) These sands were deposited in shallow marine, deltaic to interdeltaic environments and are fine to very fine grained with porosities of about 30%. Permeabilities of these sands are normally higher than 500 millidarcys.

The C sand structure map (Fig. 3) shows a broad, low relief anticline. Locations for the three proposed horizontal wells are also indicated on the map: in the order drilled - #1, #2, and #3. (The #1 well was drilled in this C sand.)

Development Strategy

Horizontal wells have been used effectively to develop shallow gas sands in the Gulf of Mexico. Expected benefits of horizontal development in the Eugene Island project included high flow rates and minimal pressure drawdown so that water coning could be avoided as long as possible.

Sand C, at 4,145 ft TVD, was the target for the #1 well. This well was designed as "extended reach", with a bottom hole location 7,450 ft west of the surface location. Pre-well interpretation indicated that the sand contained 22 ft of gas on water at the target. A 70 pilot hole would be drilled near the proposed horizontal entry point to define the top of sand and the current gas water contact. The primary advantage of the horizontal well would be to postpone water production.

Well #2 had the B sand as its target at a depth of 3,070 ft TVD. The B sand has lower porosity (&lt; 30%) and permeability (&lt; 500 millidarcys) with 12 ft of gas and no water at the target area. This sand was considered marginally economic for development with a vertical well using a gravel pack completion. The inherent high production rates of a horizontal well were needed to improve these economics. A pilot hole would also be drilled for the #2 well to define the target prior to horizontal drilling.

The #3 well's target was the A sand, which occurs at only 1,060 ft TVD.

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