Using Real-Time Magnetic Resonance to Assess Rock Quality

Overview

An operator in the Middle East wanted to evaluate an oil-bearing clastic formation composed of both shaly sands and clean sands with major grain size variations, using an oil-based mud. Historically, the operator had faced challenges in evaluating the rock quality via conventional logging-while-drilling (LWD) technologies. This was due to the variations in particle size within the reservoir not being associated with measurable changes in lithology using gamma ray, resistivity, density and neutron tools. Nuclear magnetic resonance (NMR) logs had previously been used in both the exploration and field development phases in several fields in the area for formation evaluation. However, the T₁ data while drilling method of determining rock quality in slim holes (in this case, 6 1⁄8-in. diameter) in clastic formations had not yet been attempted. 

Sperry Drilling had worked with this operator before to verify the value of T₁ data for real-time petrophysical analysis in carbonate reservoirs in 6 1⁄8-in. hole sections. Following successful deployments, they decided to evaluate the value of T₁ data in clastics in combination with density and neutron measurements from the Sperry Drilling ALD™ and CTN™ tools, and deep-reading resistivity measurements from the ADR™ tool, within a Geo-Pilot® 5200 rotary steerable system bottom hole assembly. This engineered drilling solution maximized the customer’s asset value with more reliable data for better decisionmaking with regards to completion design.

Partial Porosities and Permeability Profile Helps Operator Maximize Asset Value

Working in close collaboration, the Sperry Drilling team and the operator determined the optimum tool setup and operating parameters via the MRIL®-WD™ Job Planner. By selecting 6 ms and 140 ms cutoffs to split the T₁ relaxation spectrum, the petrophysical analysis was able to discern porosity to clay-bound water, capillary-bound water and free-fluid volumes. The NMR porosity showed excellent agreement with the conventional porosity assessment from ADL and CTN LWD measurements. Permeability was then calculated using the Coates equation, which showed streaks that represent the best rock quality, which does not necessarily conform with the porosity measurements, emphasizing that porosity alone cannot be used to determine reservoir quality. The combination of partial porosities and permeability profile helped the operator enhance reservoir understanding and was used as input into the completion design for the well.

Vibration-Tolerant T₁ Data Provides Confidence for While-Drilling Applications

Throughout the LWD operation, significant vibration and hole washout was identified. The Sperry Drilling engineered solution comprised of the T₁ data acquisition method and the MRIL®-WD™ tool design, minimized impact on data quality from lateral vibration commonly associated with this drilling environment, and the impact of borehole enlargement. From a comparison between real-time data and memory data, as well as comparing data from two different tools while re-logging a section of the well without drilling-associated vibration, the operator observed excellent correlation among all data sets. By confirming the validity of the T₁ data sets, the operator gained confidence that the measurements could be used to enhance reservoir understanding in future while-drilling formation evaluation applications.