Integrated Thinbed Petrophysics Evaluation to Realize the Production Upside and Protection Using Production Logging Post-Drilling in 2019 vs After 2 Years 2021 - Case Study in Dolphin Matured Field, Trinidad & Tobago
Authors: Gan Thomas , Kumar Vivek , Bunraj Clayton , Ramkissoon Chris 
Published: September 2022

Shell Trinidad & Tobago (T&T) is the operator of Dolphin gas field in the East Coast Marine Area (ECMA) offshore, which the field started production in 1996. The reservoirs consist of a series of stacked Pleistocene sands with clean sands and thin beds of good porosity and permeability within a three-way-dip closure against a large growth fault. In 2018, Shell T&T embarked on Dolphin Extension Campaign project of 4 infill wells to increase gas production to backfill to Atlantic LNG export and domestic market. Upon completion of the drilling campaign, a Production Logging Tool (PLT) program was immediately executed afterwards on March 2019, successfully increasing the initial production rate by another ~30% comparing to the initial production promise from the total of 4 wells campaign. After two years in December 2021, another PLT campaign was executed as part of the reservoir surveillance program which showed increased thin-beds production.

Logging-While-Drilling (LWD) open-hole was acquired as part of the wells drilling and immediately after the wells were completed and put onto production, cased-hole production logging was executed to acquire the well inflow profile information. The key Well, Reservoir and Facility Management (WRFM) risks for the Asset are sand production and water breakthrough, hence all new infill wells are required to have sand control and strict Well Operating Envelope (WOE) to ensure the integrity of the screens and sand management. It is challenging to define the WOE, even with the long historical production data and experience because of the differential depletion, differential pressures and unknown relative contributions between the clean sands and thin bed intervals. The Shell proprietary, LowReP (Low Resistivity Pay) evaluation methodology was used to evaluate the inflow profile from the well logs, which indicated an upside to well rates. Production logs were acquired to confirm the LowReP observations and define the productivity profile better.

Integrated petrophysics evaluation was performed using the LWD open-hole and cased-hole production logs. The key observations were as follows, 1) The PLT logs reconfirmed the flow from thin beds. 2) It showed that there was significantly higher than expected flow from less depleted thin beds in the wells with considerable differential depletion between clean sands and thin beds. In other wells with more uniform depletion, the flow contribution was as expected validating the LowReP model. 3) PLT logs confirmed the individual sands contribution and enabled an increase of production in all three wells to a combined total of ~30% of the campaign total production (equivalent to a new well), at a total cost of less than 2% of Project CAPEX.

It was not conventional operations to acquire cased-hole PLT logs immediately after the wells were drilled and completed. The success of this integrated petrophysics workflow demonstrated the value of information from combination of open-hole and cased-hole PLT logs to provide a complete understanding of reservoir characterization and dynamic data, especially the contribution from commingled stacked reservoirs development.