The importance of multi-discipline integration in the various phases of hydrocarbon exploitation cannot be over-emphasized. In the past, the various subsurface disciplines, within the oil and gas industry, worked in silo-like organizations which often results in a sub-optimal understanding/evaluation of the subsurface data. However, in recent times, much has been done and written on multi-disciplinary integration and its benefits particularly with respect to subsurface studies. The Zed field, which is the subject of this paper, is a predominantly gas bearing partially appraised field. The field is composed of a series of stacked sandstone reservoirs located in the Niger-Delta Region of Nigeria. Given the limited subsurface data available within the hydrocarbon-bearing areas of the field (only 2 of the 6 wells in the field penetrated the hydrocarbon-bearing sections), one of the biggest challenges of developing this field remain the high level of subsurface uncertainties coupled with the potentially low economic value of further appraisal and development of the field. In order to adequately assess these uncertainties and the economic feasibility of developing the Zed field, a detailed subsurface study involving a full re-evaluation of all potential hydrocarbon bearing sands penetrated by the wells was required. The study, which kicked off with a comprehensive integrated multi-discipline data review and quicklook evaluation, resulted in the identification of two additional reservoirs previously considered too marginal to contain substantial hydrocarbon. This paper details how the systematic, multi-discipline data integration and review of these two reservoirs helped in the identification and determination of higher hydrocarbon volumes in these reservoirs; and how this has helped in improving the economic value of the Zed field development project.
One of the key uncertainties associated with a subsurface study is structural uncertainty, especially as it relates to resource volume estimation and reservoir connectivity across a structure of interest. Connectivity and volumetrics therefore play a key role in determining the viability of projects as they have impact on hydrocarbon recovery, well count and project economics. Hence, a robust and realistic understanding of the structural configuration of an area of interest in a subsurface study cannot be over emphasised. Seismic interpretation is done to define the structural configuration of an area of interest and a key stage in the seismic interpretation process is the loop picking stage. Loop picking can be particularly challenging when there is minimal well control, poor seismic data or a plague of bifurcating loops (doublets) around the area of interest. The Zed_X1 reservoir (the subject of this paper) is a gas reservoir located in a Partially-Appraised Field (PAF) in the Niger-Delta region of Nigeria. As is typical of PAFs, the Zed_X1 field has limited subsurface data (only two well penetrations in the hydrocarbon accumulation) hence high structural uncertainty away from well control. One of the critical aspects of structural uncertainties in this reservoir is the depth of the saddle which occurs in the middle of the reservoir. On seismic, the area around the saddle occurs as doublets. Any of these doublets will translate into different reservoir volumetrics and thus development scenarios as the structure is generally flat. That is, picking the shallower bifurcating loop will generate a single culmination that can be drained by one drainage point, while picking the deeper one will translate into a deeper saddle that splits the culmination into two unconnected accumulations, hence requiring two drainage points to maximise development. The shallower arm of the doublet and deeper arm of the doublet loop interpretations were carried out and validated by wells that penetrated the structure, particularly the two wells that logged the same Gas Water Contact (GWC) on either side of the saddle. This paper documents the potential impact that Loop Picking uncertainty can have on reservoir development decisions. The full range of resource volumes and connectivity is presented by generating different scenarios in the saddle area. The impact of this on the development of the field and on project economics is also highlighted.
Considering the imminent end of the ‘easy oil’ era, the increasing demand for energy and the global push towards the energy transition, oil and gas companies are more than ever interested in sustainable ways to develop marginal and complex hydrocarbon fields economically, through the application of technology and maximization of data analysis. In small partially appraised fields where the cost of drilling an appraisal well could derail the project economics, it becomes necessary to sweat the limited data available for reservoir modelling. The uncertainty analysis must be robust enough to ensure that the adopted field development strategy would yield a positive net present value despite the wide uncertainties associated with the field. The conventional workflow for subsurface uncertainty modelling involves defining the uncertainty ranges of static and dynamic reservoir parameters based on a single reservoir model concept. This paper focuses on a marginal field case study where the multi scenario modelling approach was adopted. This approach considered alternate reservoir geologic concepts based on different interpretations of the reservoir architecture, taking full cognizance of the available data, reservoir uncertainties and regional geology knowledge. Field Alpha is located onshore of Niger Delta in Nigeria. The geologic setting consists mainly of multi-storey, complex channel-belt systems, incising through Shoreface deposits. The reservoir of interest is an elongated structure with only two well penetrations located at the opposite distal part of the structure. The key reservoir uncertainties are reservoir structure, architecture, connectivity, and property distribution. Two possible distinct architecture were interpreted based on regional correlation and seismic. This paper focuses on how the interpretations and other information informed a robust development strategy that yielded significant (30 %) reduction in development cost and positive net present value.
Nigeria's gas policy vision to be an attractive gas-based industrial nation implies that we need to continuously harness the abundant gas resources in the subsurface. This may imply that many more gas projects will be required to meet the country's aspirations. Pressure depletion under natural depletion in gas development are usually much more than in oil development. There may also be compression as part of maximising gas depletion which would lead to even further pressure depletion at end of life compared to oil development. It is therefore imperative to assess the impact of pressure depletion on Top Seal Integrity. Top Seal failure can lead to loss of reservoir-fluid containment with resultant uncontrolled flow of fluids (liquids or gases) from the reservoir into the seal or into the deep overburden, and then upwards due to reservoir pore pressure or buoyancy effects which may manifest as internal blowout. ZERN field is a partially appraised field that has been identified for gas development. We have a fair understanding of our reservoir response to depletion having been producing oil reservoirs for over 50years; however, gas production is somewhat new and there are no mature analogues for benchmarking in SPDC. Mohr coulomb shear failure criteria was used to assess the risk of top seal failure. A set of deterministic scenarios were built integrating information from planned start of field production to predicted end of life and incorporating realisations of rock strength parameters from ZERN field and analogue field. Three pre-production scenarios were coupled with nine post-production scenarios for each depleting reservoir. The resulting Mohr circle envelopes for the different scenarios were analysed per reservoir to arrive at best engineering judgement for de-risking top seal integrity for the ZERN field.
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