Minimising Reservoir Top Structure Uncertainty through Integration of Seismic Horizon Well-Tied Top Interpretation and Geosteering Distance-to-Bed-Boundary Calculation from Azimuthal Resistivity Tool: Case Study from Offshore Thailand
Authors: Phettongkam Numan , Ngo Hien , Ruangsirikulchai Arisa , Somwangsombat Ngamwilai , Nasomsong Rutchanok , Visessumon Saris , Jankham Kampanart , Loviruchsutee Pakorn , Gongsakdi Nitikorn , Chommali Prayuth , Dovgopolyy Viktor , Rodriguez Jesus , Sitthivanich Anis 
Published: February 2023

Reservoir top structure interpretation through seismic usually associated with uncertainty especially when sparse number of wells are available. This leads to uncertainty in gross rock volume and oil-in-place calculation. It is quite common with reservoir top interpretation picking varies significantly from optimistic to pessimistic case especially at the point distant away from wellbore.

This paper integrates the geosteering distance-to-boundary (DTB) calculation from horizontal well drilling to minimize the uncertainty window of top interpretation from surface seismic to deliver more accurate remaining oil-in place.

During the geosteering operation, seismic horizon interpretation of reservoir target sand with multiple scenarios in offshore field was compared with the distance to boundary calculation that map the reservoir top structure with azimuthal resistivity tool. Prior to geosteer horizontal well in the oil-rim reservoir after long production, pilot hole was drilled to update the current fluid contact. Reservoir top interpretation is revised by including horizon top structure tied to well data from the pilot hole and existing wells in the area.

The revised top structure which includes the pessimistic and optimistic case generally has significant difference in remaining OIP. Upon geosteering horizontal well, DTB calculation from azimuthal resistivity tool was capable to map the top reservoir boundary in real-time with reference from the seismic top interpretation from heel to toe of the lateral with good quality mapping of entire top reservoir structure.

Distance to boundary (DTB) with good quality of top reservoir structure mapping throughout the section tracked closely to optimistic case of seismic top interpretation of target sand. Geosteering with azimuthal resistivity with RSS directional tool mapped the top of the reservoir on the upside and OWC from the downside up to 4.8 m away from the tool. As the top is expected to dip down and intersected the trajectory at multiple scenarios, the mapped boundary provided insights of gentle dipping trend, suggesting the top of reservoir could extend laterally longer than pessimistic case. With the result closer to optimistic case, the calculated remaining OIP volume calculated was on the optimistic side with significant difference, larger volume, from pessimistic case of OIP calculation.

Integration of seismic top interpretation with geosteering distance to boundary application in lateral well could minimize the uncertainty in reservoir top horizon interpretation. The workflow enabled to place the well in the sweet spot while managing the uncertainty of top structure. With production from horizontal well, the well production rate is expected to increase to 2900 BOPD for initial oil production compare to below 1000 BOPD prior to horizontal well drilling. This workflow could be applied for future lateral well with high uncertainty of the target to place the well in sweetspot for field life extension and production optimization.