High-Confidence Vertical Positioning for Extended-Reach Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 151441, ’High-Confidence Vertical Positioning for Extended-Reach Wells,’ by Nelson Mohammed, SPE, and Michael Terpening, SPE, Schlumberger, prepared for the 2012 IADC/SPE Drilling Conference and Exhibition, San Diego, California, 6-8 March. The paper has not been peer reviewed.
Establishing confidence in the vertical thickness of the hydrocarbon column is critical in development-project economics. True-vertical-depth (TVD) uncertainty associated with the hydrocarbon column, in the absence of vertical-appraisal-well data, may be dominated by the propagation of inclination error in nonvertical or horizontal pilot wells. The challenges associated with establishing reliable hydrocarbon-column height can be extreme if high-step-out extended-reach (ER) wells are the primary source of appraisal data. Also, thin reservoirs targeted by ER wells often have a limited vertical target on the basis of standard TVD-error-propagation models. Three complementary elements were integrated to improve the TVD uncertainty associated with ER well paths.
Application Challenge
The application was made in an offshore field near Sakhalin Island in eastern Russia. The structure has several appraisal wells drilled by various operators through the 1980s. A recent development-drilling campaign with ER wells was conducted that enabled evaluating and testing of the proposed high-confidence error models.
Vintage appraisal wells proved the presence of oil and provided information on the hydraulic regime with flow tests. However, there was no full penetration in a single well of the hydrocarbon column in the main sands, resulting in large gas/oil-contact (GOC) and oil/water-contact (OWC) uncertainties that precluded an economically reliable measure of oil-in-place volumes. The quality of available seismic data was poor, and environmental and economic limitations existed for upgrading data or further appraisal drilling to address uncertainties.
The chosen development option was to drill from a shore location and use extended-reach-drilling (ERD) techniques with horizontal production holes to reach the reservoirs. Hence, certainty of the OWCs and GOCs was essential for proper placement of the horizontal production holes within the oil column. The vertical uncertainty associated with ERD would need to be minimized to assess the hydrocarbon column and to place the horizontal production wells properly.
As Fig. 1 shows, the first two production wells were designed to use separate pilot holes, downward through water (Well-1) and upward through gas (Well-2), to acquire formation-pressure-while-drilling data to characterize the hydraulic regime better. The primary objective was a direct determination of the fluid-column contacts, or an accurate inference of the contacts through a well- defined pressure-gradient profile.
TVD Assurance Through Synergy
Redundant data from two separate but comparable inclination sensors in the same bottomhole assembly (BHA) [in the measurement-while-drilling (MWD) and rotary-steering-system (RSS) tools, respectively] were used to permit a statistical reduction in the error from multiple independent measurements. Because the sensors are in separate sections of the same BHA, the data enable a robust cross check to ensure that the sag corrections for each sensor are simultaneously consistent.