The Thin End Of The Test: Operational Procedures For Maximising Successful Formation Pressure Tests While Drilling In Thinly Bedded Sands
Authors: Urlwin Benjamin , Orr Mike , Kaur Amrith 
Published: March 2015

Formation pressure testing while drilling (FPTWD) is an established method of obtaining detailed reservoir pressure data during the drilling process and in operational settings that have very limited formation evaluation alternatives. With many companies, obtaining such data is an integral part of reservoir management strategies, enabling the monitoring of production and injection performance as well assessing reservoir connectivity on a field-wide scale, and in a D frame. FPTWD removes operationally challenging and time-consuming wireline or pipe-conveyed evaluation options that can add unacceptable risk and cost to a project. Obtaining this data as a part of the active drilling phase not only enables a reduction in rig time required for reservoir evaluation, but it also provides an operator valuable information that can impact decisions made on mud weights and casing points while the well is being drilled.

FPTWD testing in reservoirs dominated by thin-bedded horizons has traditionally been a hit-and-miss affair, with uncertainties in depth control resulting in a high percentage of tight tests because of pad placement within a shale layer rather than a reservoir horizon. The situation is exacerbated by the automated parameters necessary in a FPTWD tool, whereby tight tests still require the full time interval to complete, but yield no useful pressure information.

This paper outlines procedures and techniques successfully applied on turbidite sands in highly deviated wells in deep water fields of Sabah, Malaysia. Utilization of pipe stretch corrections, real-time density images, caliper readings and variations in testing methodology all proved useful in increasing the number of successful tests recorded. By analysing the time required to obtain a valid, three-drawdown test, a reduction of individual test times was made, enabling the decreased overall operational time required.

Introduction

Turbidite sands make up many of the most important reservoirs of Malaysia. Characterising the thin sand layers can be challenging, because they fall below the resolution of most conventional logs, giving responses that can be highly influenced by surrounding shales (Fig. 1). Various studies were conducted on how to accurately determine key reservoir parameters such as porosity, permeability and resistivity in these layers, including Majid et al (2012), however less work was done on analysing procedures and methodologies utilised with FPTWD tools to determine how to increase the percentage of successful tests in such runs. FPTWD is important for establishing baseline formation pressures in unexploited layers, and determining communication and depletion levels in horizons where production and injection occur. Operationally, determining the pressure of a newly drilled layer is critical to confirm mud weights required to remain over balanced with respect to the reservoir pressure, or conversely, to be sure not to be too over balanced, and therefore avoid fracturing the reservoir. Recording formation pressures in a real-time drilling scenario enables real-time mud weight changes to be made as well as real-time assessments on whether these changes can be made safely (given the pressures of layers already drilled). In some cases, the pressure information will show that an intermediate casing run is required in order to safely drill deeper, making it vital that this data is obtained while drilling. Accurately positioning a test probe, near the bottom of thousands of metres of elastic drill pipe, onto a relatively thin sand layer is extremely challenging, and can lead to tight tests if the probe is positioned on the surrounding shale. Detailed procedures must be developed and followed to maximise the chances of getting a good test in such an environment. These must also be actively updated, through continued analysis of test type and duration, to optimise test times, and therefore, reduce the amount of rig time taken.