Multiple Drain Point Approach in Thin Multi-Layered Carbonate Gas Reservoir, A Case Study of Onshore Extended Reach Drilling in UAE

Abstract

This case study is of drilling extended reach 6" lateral (more than 7,000 ft) in thin multi -layered carbonate gas reservoirs with a novel approach of tapping the multiple target reservoir units in dual points starting from top reservoir unit to the base unit and placing the well back to top reservoir unit in steps. The well trajectory was planned with Top-Bottom-Top (TBT) approach, starting from top to bottom layers and steering back to the top layer in stair-step trajectory. The MWD-LWD BHA was selected to continuously monitor the porosity to avoid exiting from porous subunits – owing to the thinness of sweet spot in reservoir subunits with the range of 4 to 5 ft. only. Azimuthal Resistivity tool with LWD triple combo was used while geosteering the well to assess and map the subunit boundary as there is good resistivity contrast between porous subunits and bounding stylolite. LWD Pressure Formation Tester was used to record the current reservoir pressure in the target reservoir for the purpose of optimization of the mud weight to avoid the risk of differential sticking due to higher overbalance. The differential sticking was experienced in the previous wells due to higher mud weight and overbalance. Hence, mud weight optimization helped to drill more than 7000 ft of 6" horizontal section with a complex stair-step well trajectory design. The first 5,000 ft of horizontal section have been drilled successfully using distance to boundary Azimuthal resistivity tool in addition to density-neutron tool. While in the remaining of 2,000 ft horizontal length of drain hole, the radioactive source tool was replaced with source less BHA of azimuthal resistivity and sonic tools. The reason for replacing source tool with sourceless tool is the risk of string stuck up with radioactive source in the BHA. This may be caused by complex stair-step well trajectory, reservoir pressure uncertainty and any down hole complication. The target reservoir identified for this approach has low average permeability of less than 1 mD with limited sweep area. The target is thin stacked reservoir subunits of thickness ranging from 4 to 8ft. The Subunits porosity range is 3 to 17% and are distinctly bounded by thin non-porous stylolite. The risk of drilling with complex well trajectory was handled by constant maintenance of dogleg severity (DLS) less than 2deg/100ft. Each subunit was targeted with very gentle inclination and inter-bedded stylolites were cut with higher inclination to achieve more than 90% of reservoir contact. The project has resulted in reservoir characterization in selective areas with selective drain. Being laterally heterogeneous, the Top-bottom-top approach provides the scope of selective drain in the reservoir along the well path. A higher production performance is expected from this approach as each subunit was targeted according to their reservoir properties. This case study proves to be novel especially in tight reservoirs with limited drainage area, where cost have been optimized by reducing the number of wells to be drilled by tapping the reservoirs in dual drainage points with a single well.