Successful Application of a Novel Mobility Geosteering Technique in a Stratified Low-Permeability Carbonate Reservoir
Authors: Al-Suwaidi Salem H. , Lyngra Stig , Roberts Iwan , Al-Hussain Jihad , Pasaribu Ihsan , Laota Abdalla S. , Hutabarat Sanggam 
Published: September 2013

A novel hybrid steering methodology employed for a horizontal development well in Saudi Arabia makes use of three independent and mutually supportive geosteering concepts: mobility steering, a new real-time true stratigraphic thickness (TST) path-integration method, and distance-to-boundary mapping. This technique, employed for a horizontal development well in Saudi Arabia, proved the feasibility of mobility steering in a permeable zone of limited thickness, without any characteristic log markers separating the target zone from the low-permeability rock located above and below, supported only by the distance-to-boundary mapping and TST path integration techniques.

A mature giant Middle Eastern field contains a previously undeveloped stratigraphically trapped carbonate oil accumulation discovered in the 1980s. Several delineation wells were drilled at that time, but no recent penetrations exist as this reservoir is located deeper than the producing horizons. The reservoir of interest contains high-quality oil, but the rock is predominantly &lt;1 millidarcy (md) permeability.

In early 2012, the first development well was spudded. A 30° slanted pilot hole was first drilled and extensively logged. The well data confirmed the low permeability, but also demonstrated limited development of higher permeability rock in vertically restricted zones within a few feet of the top of the reservoir.

A horizontal development sidetrack well was subsequently drilled using advanced real-time logging-while-drilling (LWD) measurements to steer along the thin higher-quality interval. Potential practical obstacles in achieving optimum well placement were the lack of traditional log signatures or markers correlating with permeability, uncertainty about lateral and vertical extent of permeable rock, and no database of existing horizontal well logs and images in this zone.

The LWD measurements required mobility (permeability/fluid viscosity) measurements, a continuous means of determining the attitude of the strata to steer parallel while maintaining standoff from the top boundary of the reservoir, and a basic evaluation petrophysical LWD suite.

This LWD string —, the longest successfully used on a kelly driven rig located onshore in Saudi Arabia — consisted of a total of five LWD tools plus an instrumented rotary steerable tool for precise trajectory control. Careful attention was paid to operational planning and process workflows, detailed LWD assembly design, and testing, feasibility studies on measurement responses, data acquisition planning, and directional drilling.

The horizontal lateral, which achieved 100% net-to-gross (N/G) in reservoir comprising &gt;= 5 millidarcy/centipoise (md/cp) mobility, was geosteered within a narrow, &lt; 2 feet (ft) thick target interval without any distinguishing geosteering log markers. The target interval was identified and verified using the mobility measurements from the LWD formation tester tool. Some relatively high real-time mobilities were observed along the lateral, exceeding the expectations set based on the offset pilot hole data.

The horizontal development sidetrack was put on production in late 2012 producing at an oil rate of approximately (~) 4,000 stock-tank barrels per day (STB/D).