Reducing Geologic Uncertainty By Combining Seismic Models With Real-Time LWD Measurements: Mars A-8 Case Study
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 134181, ’Combining Seismic Models With Real-Time LWD Measurements to Reduce Geologic Uncertainty: A Case Study of the Mars A-8 Extended Reach Well,’ by Jeff Weiland, SPE, Matt Cuttitta, SPE, and Gerard Soto, Shell, and Matt Wandstrat, SPE, Brian Donadieu, SPE, and Segun Jebutu, SPE, Baker Hughes, prepared for the 2010 SPE Annual Technical Conference and Ex hibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed.
While surface-seismic data are used to evaluate potential targets, they lack the resolution needed to place those targets reliably. True-vertical-depth (TVD) and true-vertical-thickness (TVT) synthetic ties created from real-time logging data can improve that resolution. By use of TVT, the synthetic ties match the seismic model better, enabling updates to the well path and precise placement inside the target. Also, multiple velocity models are incorporated to obtain better time/depth relation-ships and to predict pore-pressure anomalies better.
Introduction
During the summer of 2006, the six-block unit covering the Mars field was set to contract and the shallow exploration well just drilled in the area had yielded marginal results. Mississippi Canyon (MC) Blocks 850 and 851 were set to expire if production could not be established. Shell had identified potential geologic targets on Block 851 but had not developed a well plan. To establish production in time to maintain control of the lease, it was necessary to drill the well from the Mars platform, which required an extended-reach well. Geologic uncertainties and mechanical complexities created a challenging well.
Background
The Mars field, shown in Fig. 1, is in the MC area of the Gulf of Mexico (GOM), 130 miles southeast of New Orleans. The first discovery well was drilled in 1989 on MC Block 763. The field was developed from a 24-slot tension-leg platform (TLP) installed on MC Block 807 in May 1996 in 2,940 ft of water. Production began in July 1996 and peaked in June 2000 at 208,000 BOPD and 217 MMcf/D of gas.
Primary production from Blocks 763, 806, and 807 in 1996 established the unit. Block 762 was set to expire in 2006, followed by Blocks 850 and 851 2 years later. In June 2006, Well MC 762-6 came on production, establishing MC Block 762 in the unit. However, with marginal results from the subsequent MC 850-1 exploration well in mid-2006, the future of Blocks 850 and 851 remained in doubt. Shell used the last available slot on the TLP to execute the MC 851 A-8 exploration well.
Need for Extended-Reach TLP Well. While subsea vertical wells would be easier and faster to drill, the time needed to build subsea manifolds, construct subsea trees, and lay flowlines was considerable. With production required to hold the leases, drilling extended-reach wells from the Mars TLP was judged the best option. Although drilling and completion operations would be more expensive and more complicated to execute, production could commence immediately after completing the well.