Application Of Distributed Horizontal Drill Stem Pulse Tests To The Appraisal Of Naturally Fractured Resource In The Devonian Clair Group

Abstract

This paper describes the first known offshore application of distributed horizontal pulse testing. This technique appraises the deliverability of naturally fractured resource, using only a single penetration and Drill Stem Test (DST) string run. Central to the design is the ability to collect near real-time interference data at a known minimum length from a drawdown interval. The pulse test is distributed in that the signal and observation zones can be swapped on demand from surface, acoustically via sleeve. BP successfully applied this technique to two appraisal wells in their 2014 offshore drilling operations.The dual zone DST pulse testing method is a new approach to the appraisal of naturally fractured reservoirs. It was developed to create a real-time interference dataset outwith the active production interval, i.e. within a passive zone. The formation is produced (and rate data are measured) in two spatial locations and across two known length-scales (by swapping the active and passive zones prior to commingling). Pressure diffusivity can thus be calibrated to data measured in two spatial locations of the same reservoir, and not just one, as per a conventional test design, i.e. enabling a history match of the pressure response of two lateral zones from a pulse signal in one. The horizontal aspect is achieved via high-angle well, drilled sub-parallel to unit bedding (Figure 1).With the double staging approach (upper and lower zones), which has been developed for fractured reservoir appraisal studies, three tests were successfully performed in each appraisal well: two partial penetration tests on discrete short intervals (DST#1a, DST#1b), and one final test on a longer interval that included both of the short intervals (DST#1c). Results of this application have demonstrated that pressure diffusivity can be derived from pressure data that are measured simultaneously in two spatial locations, within and outwith an active production interval. This has proved particularly useful for reducing the degrees of freedom in reservoir model identification. The paper concludes that appraisal of naturally fractured reservoir might be sub-optimal in a DST design where drawdown and observation data are limited to a singular inflow zone only. This is the first known application in the industry where one DST run has successfully yielded six unique appraisal data types, gathered simultaneously at both intra/inter zone and commingled lengthscales. These data are conventionally not gathered in a single zone design, i.e. without the ability to selectively inflow and monitor pressure in each discrete lateral zone. Consequently this technique has significantly improved the description of both static and dynamic reservoir properties and reduced development uncertainty, all at a relatively low incremental cost.