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Abstract The giant Khazzan gas field, onshore Oman covers an area of 2800 km2 and has been on production since 2017. Structural depth is a key uncertainty particularly in the flanks of the field that have fewer wells and low structural relief. With a shallowing GWC in these areas, the water risk increases and consequently, the recoverable gas and associated liquid content have become key development considerations. A well with a vertical seismic profile (VSP) was planned to improve the velocity control, narrow down the structural uncertainty and subsequently de-risk the resources and optimize the development plan in one of the areas where development is sensitive to the depth of the structure. Geomorphologically, this area is in a part of the field where there is a salina (weak evaporites) at surface.
The VSP process was split into three phases: planning, operations, and analysis/results. The planning phase focused on the survey design to achieve the VSP objectives by means of: a) reviewing existing VSPs, pre-survey modelling and working closely with technology providers, b) salina engineering feasibility followed by vibrator test on the salina and c) pre-job preparation via conducting ‘shoot the survey on paper’ and ‘risk assessment’ to produce a main plan with back up options that included key decision points.
The planning phase concluded with the following recommendations: a) VSP collection using fiber optic hybrid logging cable and Distributed Acoustic Sensing (DAS) technology, to efficiently collect velocity information (although risks to this ‘first of a kind’ operation in the area were mitigated by mobilizing a conventional VSP tool); b) acquire Zero offset (ZO) and walkaway (WA) VSP with 4 km maximum offset to measure anisotropy and velocity away from the well; c) acquire WAVSP along two perpendicular lines to estimate azimuthal anisotropy related to current stress regime; d) use an existing road for the primary WAVSP line and build an additional 4 km road for the second line to mitigate soft conditions in the salina. The salina poses risks to operating vibrators while driving and sweeping and these risks needed to be managed. During the acquisition phase, operational expertise on-site with co-location of the company decision makers would provide around-the-clock support to improve efficiencies and respond rapidly to any operational challenges. One contractor was chosen to provide all VSP services to reduce job complexity and minimize interfaces.
VSPs acquired using fiber optic technology are becoming more common but this trial is the first in block 61 and possibly in the area, to use a hybrid optical-electric logging cable to record both zero-offset and walkaways. The ZOVSP DAS data was collected using a formation tester tool in the tool string. This showed inferior data quality due to difficulties in coupling the cable to the borehole wall and casing, which unfortunately limited the success of the trial. Only limited sections of the cable were coupled and could be used for first break time picking. This triggered the backup option, a conventional geophone array deployed with the hybrid cable, to successfully record the ZOVSP and two WAVSP. DAS data was recorded simultaneously, to try using the intervals with good coupling later during data processing.
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