Ultradeep Resistivity Inversion / Geomapping Technology Addresses Challenges in New Zealand's Offshore Mature Oil Field
Authors: Kharitonov A. , Clegg N. , Ma J. , Elicia R. 
Published: October 2024

Placing horizontal infill wells in New Zealand's mature fields targeting formations that are normally thinner than the primary units and feature less optimal petrophysical properties e.g. low resistivity contrast, uncertain continuity due to water migration or channel complexity poses significant problems. To address this a novel Ultra-deep resistivity inversion / geomapping technology was deployed to provide greater reservoir understanding in complex geology and clarify the stratigraphic location for the well placement.

Extensive modelling was conducted using nearby offset well data and seismic grids to identify the optimum setup for real-time geomapping operations using both standard formation evaluation data and an ultradeep azimuthal resistivity service. The primary approach is to generate multiple inversion results for the given resistivity profile defined by the offset well profile using various frequency combinations and spacings to simulate inversion results expected during real-time operations. Once drilling commenced, the real-time inversions from the Ultra-Deep Resistivity (UDR) were qualified against conventional FE data including deep azimuthal resistivity and azimuthal density images. Inversions from various spacing and frequencies are also crossed checked with each other to improve the confidence level on the features identified including top and bottom boundaries.

The challenging geology setting in the Taranaki basin of New Zealand presents both fluvial / estuarian channel complexes and marginal marine settings where water flooding from years of production has increased the complexity of the adjacent reservoir character. Ultradeep resistivity inversion has proved to be effective at mapping deep into the formations to assist understanding of the well's stratigraphic position and was able to not only identify the well paths position within the complex reservoir, but it also maps the adjacent formations which provided additional assurance to the understanding of the wells’ stratigraphic positions. When intersected, the reservoir boundaries or facies changes within the reservoir, the ultradeep resistivity inversion results tally with the shallower data from conventional sensors in the BHA. This provided increased confidence in the inversion results at distances further away. This enhanced understanding of distance to the boundaries allowed the subsurface team to place the well path in the optimal position within the target reservoirs. Apart from strategic geosteering decisions based on seismic data and recent offset wells, most geosteering decisions were made utilizing ultradeep resistivity inversion canvases. High quality of inversion results with matching conventional logs quickly built-up confidence within the client subsurface team and promoted the active interpretation of the inversion results. Integration of the full suite of geosteering/geomapping information from shallow to ultradeep measurements allowed the operator to gain a better understanding of the target reservoir drilled and optimized placement of the wells.

This paper presents case studies of several horizontal wells geosteered in complicated geological settings in some of the mature fields in the Taranaki basin of New Zealand. These wells would be rather difficult/challenging for the operators without the ultra-deep resistivity inversion technology. Successful completion of these wells proved this technology to be of great benefit to the operator to gain better understanding of their existing assets.