Combined Approach Improves Fault Description for Horizontal-Well Geosteering
Authors: Carpenter Chris 
Published: January 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191695-18RPTC, “Complex Approach to Fault Description While Geosteering for Maximization of Reservoir Contact in Horizontal Wells in West Siberia Oil Fields,” by Vladislav V. Krutko, Tatyana A. Yurkina, SPE, and Dmitriy Y. Kushnir, Baker Hughes, a GE Company, and Valery B. Karpov, RITEK, prepared for the 2018 SPE Russian Petroleum Technology Conference, Moscow, 15–17 October. The paper has not been peer reviewed.

This paper presents an interdisciplinary approach to the description of tectonic dislocations made on the basis of interpretation of seismic data, petrophysical analysis of well-logging data in horizontal wells, and inversion of a multifrequency propagation tool. A consistent approach to fault identification and description is presented on the basis of seismic surveys and logging-while-drilling (LWD) data in horizontal wells in a western Siberian oil field.

 

Seismic Methods of Tectonic-Fault Interpretation

Estimation of seismic methods of fault detection was performed on materials acquired from one of the fields in the Frolov oil-and-gas district. The observed territory of the oil field is characterized by complex geological structure—namely low effective reservoir thickness, thin layering of sandstones and silts, low porosity, low-permeability reservoir zones, and tectonic block structure.

When drilling in reservoirs of low thickness, knowing the precise position of the horizontal wellbore relative to the structure is critical. The basis of drilling planning is a structural map of the reservoir top. After field jobs were complete and the results of the wide-aperture 3D seismic survey of the considerable refinement of the top structure of the reservoir were obtained, the morphology of all earlier identified structural forms was completed. A faulted and blocked principal model of the reservoir was created that formed the basis for a well-pad positioning scheme, as well as for usage in horizontal-well drilling.

Horizontal Well 2301G was planned in the northwest region of the field. For reservoir-model construction, a structural map prepared from seismic-survey interpretation was used. An outcrop of the structural map with Well 2301G in place is presented in Fig. 1.

A rare well network in the drilling area led to high uncertainty regarding structure parameters and risk of penetrating a fault. As an offset, Well 75B3 was used (Fig. 1). In this offset well, the horizon AC3 represented a thin layering of silt, shaly sandstone, and tight rocks. The thickness of the AC3 horizon was 13.2 m. During the drilling of the transport section, the wellbore of Well 2301G penetrated the top of the AC3 horizon at 1912.12-m true vertical depth, which coincided with the structural surface prognosis made on the basis of seismic data.

The blocked model of the AC33 horizon is complex; the vision of the structure of the target reservoir can change significantly while drilling and penetrating the reservoir. According to seismic data, the possibility exists of crossing three faults along the planned horizontal section of Well 2301G.