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Content archived on 2022-12-27

DEVELOPMENT OF A TECHNIQUE TO EXPLORE SUBSURFACE GAS-WATER CONTACTS

Objective

To examine the geological and geophysical problems of detecting and validating subsurface gas-water interfaces in hydrocarbon reservoirs as an aid to the direct detection and mapping of gas reserves. A number of known gasfield examples in northwest Europe were examined and compared to other possible, but unproven, examples. In particular a horizontal subsurface seismic feature in the Kish Basin, offshore Ireland, was detailed by a seismic survey, and later penetrated by an exploration well, at which time geological-geophysical studies were made of the critical rock section.
100 km of additional reflection seismic data were obtained to examine a possible gas-water interface in the Kish Basin. At the same time a maturation study of samples from the existing Amoco 33 /22-1 well and related areas was aimed at a study of source potential. Processing of the Kish Basin lines demonstrated that the reflection was probably not a fluid-contact reflection. Petrographic and electron microscope studies of samples from the nearby shell 33/21-1 well showed that the potential Triassic reservoir demonstrated high average porosities with only limited secondary mineral development to inhibit permeability.
As part of a wider documentation of gas-water contacts about 20 published and unpublished examples have been collected worldwide.
These examples are from a wide variety of geological settings and show that for recognition of a fluid contact refection it must be a positive reflection, observed in horizontal or near-horizontal attitude, in a trappint position, against abackground of moderate dip.
Drilling of the Kish Basin 33/17-1 well took place in May-June 1986. The well was plugged and abandoned as a dry hole at 6600 feet (2012 m). Results from the well substantially confirmed the geological prognosis for the structure and confirmed that the anomalou flat refector on the prospect was not a fluid-contact reflection.
The study showed that where seismic quality is good and reservoir thickness/gas column are adequate, then careful acquisition and processing can allow a determination of whether the gas-liquid contqct is horizontal and meaningful estimates of reservoir porosity and gas column. Confirmation of porosity by amplitude study is worth a test and may on occasion indicate, by comparison with velocity data, a component from a porosity step caused by differential cementation.
Where such a step is separated from the present gas-water contact it represents a fossil gas-liquid contact, a study ofd which can reveal the history of folding and tilting since gas emplacement and may allow definition of the commercial base of the gas accumulation. Improving seismic techniques allow increasing recognition of gas-liquid reflections in poor-quality data areas (for example from reservoir porosities of the order of 15 %). Nevertheless, hoaxes can occur, and the report describes a number of these. In terms of exploration philosophy the report points out that a gas-liquid reflection is the most significant indication of hydrocarbons that is available in exploration and that it should be exhaustively investigated prior to drilling.
The absence of a reflector may also be of utmost importance and factors influencing this are analysed. The study of gas-liquid reflections is useful not only in the positive sense of confirming the presence and volume of gas, but also in the negative sense of avoiding dry holes.
Phase 1: carefully monitored seismic acquisition (with large volume water guns) and processing of reflection seismic data over the Kish Basin example and the production of time and depth maps of the principal reflecting horizons. Delineation of the gas water contact. Maturation studies of existing well samples.
Phase 2: detailed lithological/petrological studies of the two existing Kish Basin wells and analysis of the possible gas-water contact in the Basin. Documentation of known gas-water examples from other areas world-wide. Drilling of a deep exploration well in the Kish Basin and study of the results.
Phase 3: complete analysis of Kish Basin results.
Studies of three gas-water contacts with reprocessing of seismic data and detailed analysis of electric log and petrological data. Phase 4: final report preparation.

Call for proposal

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Coordinator

KISH DEVELOPMENT LTD
EU contribution
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Address
162 CLONTARF ROAD CLONTARF
3 Dublin
Ireland

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