Advanced Formation Testing in OBM Using Focused Fluid Sampling for Producibility Evaluation in Mature Fields

Abstract

Wireline formation testing provides formation pressures, high quality samples and fluid identification/characterization. In addition, it can provide information for reserve assessment and producibility estimation. In this paper, we present comprehensive formation evaluation case histories with formation testing utilizing a focused sampling probe in wells drilled with Oil Base Muds (OBM) in mature fields. Due to OBM and low mobility sections, a new focused sampling device was utilized for effective formation testing and sampling purity. One case history demonstrates confirming remaining oil saturation. Conventional open hole and Nuclear Magnetic Resonance (NMR) logs were run for formation evaluation, and fluid saturations. Gas and remaining oil saturation were obtained from 3D NMR analysis. Sampling lab results and real time analysis of gas compositions are also compared for verification and confidence. Two field examples in low porosity/low mobility zones are presented showing the identification of mobile fluids. It is shown that the existence of mobile oil could have been missed without effective formation testing practices. The third field example of formation testing with low porosity as well as low resistivity is provided for the identification of mobile oil. Flow was enabled from low mobility zones along with low rate pumpout that would not be possible with traditional probe and pumpout devices. The final field example of water sweep evaluation in high permeabilitiy/mobility zones demonstrates using formation testing as a means of reservoir monitoring. Introduction Recent advances in wireline formation testers enabled the determination of several fluid properties, including fluid compositions in real time. The applications of wireline formation testing has been discussed in a number of publications concerning Downhole Fluid Analysis (DFA)1–4. It has been shown that compartmentalization and compositional variation can be positively identified. This evaluation can have significant impact on facility sizing and economics of field development of new reservoirs. In addition, mini-DST or Interval Pressure Transient Testing (IPTT) can be carried out at DFA stations to provide more representative and accurate mobility/permeability distributions of reservoir layers5 for reservoir characterization. Mature reservoirs can have different objectives and challenges for efficient formation evaluation and reservoir management. As gas cap expands and active aquifer water moves in heteregenous systems, formation pressure profiles can be more complicated. Evaluation of saturations/remaining oil determinations becomes quite important for the determination of sweep efficiency in mature reservoirs. Low porosity/low permeability sections can be more challenging to evaluate and these sections can still have considerable hydrocarbon potential. Although, OBM can be challenging for contamination quantification during sampling operation, it has been shown that optical spectroscopy can be quite effective to overcome contamination challenges6. The challenge can be much more in the case of determining small amounts of mobile oil in a gas expanded zone, drilled with OBM. A focused sampling device is introduced to overcome this challenge by isolating small amount of filtrate that can mix with sampling line. Lab results of the first field example demonstrated that these objectives were achieved.