Physics-Based Well Design - Engineering the Well for Production
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 10705, "Physics-Based Well Design - Engineering the Well for Production," by B. Kline, SPE, B. Dale, SPE, R. Pakal, SPE, S. Clingman, M. Asmann, and D. Rosenbaum, SPE, ExxonMobil, prepared for the 2005 International Petroleum Technology Conference, Doha, Qatar, 21–23 November.
Ensuring long-term well integrity and optimum completion performance is essential for economic development of assets developed with fewer, more-complex wells. The full-length paper describes unique, physics-based modeling capabilities used during well planning, design, and production to deliver optimized well performance over the life of the well. The modeling capabilities integrate fundamental physics to provide a more accurate determination of well-performance technical limits.

Background
In the past, top performing fields produced thousands of barrels a day from each of dozens of wells with completion lengths spanning tens to hundreds of feet. Today, fewer wells are used, each producing tens of thousands of barrels a day from longer and more-complex completions often spanning thousands of feet in more-technically-challenging environments. Obtaining superior well performance requires an understanding of the physics that controls well production as well as new technologies that use physics-based knowledge.

Well Operability
Well-integrity-related technical limits called well operability limits (WOLs) establish the boundary between safe well production operations and completion failure resulting from rock compaction/dilation, sand production, and other geomechanical-related well-failure problems commonly associated with reservoir depletion and/or near-well pressure-drawdown effects. The technologies that establish WOLs combine practical application of stress-/strain-based reservoir management with traditional pressure-/flow-based reservoir management. WOLs are developed by use of advanced finite-element-analysis (FEA) models based on coupled flow/deformation theories. These FEA models were developed by use of an integrated framework that uses well logs, geological models, reservoir-simulation models, and geomechanics characterization of reservoir-/overburden-rock core samples. Results from these models are translated into simple field operating conditions. Fig. 1 shows an example of how producing conditions from permanent-downhole-gauge data or other real-time downhole measurements can be compared to the WOLs to determine current well-integrity status and proactively optimize well operating conditions in day-to-day production operations.
Compaction. Compaction WOLs were created in response to experience in deepwater Gulf of Mexico assets that experienced early compaction-related well failure. Application of these WOLs in production operations within this field has resulted in no additional well failures while increasing producing volumes by increasing drawdowns far in excess of analog limits based on empirical maximum-production laws and offset-field experiences. Geomechanics-failure-related WOLs have been adopted in other assets around the world to provide physics-based production guidance to avoid well failures while meeting aggressive production targets. While the absence of well failures has obvious economic advantages, the largest benefit WOLs have provided is the confidence to produce wells more aggressively than they would have been produced without the quantified technical limit.
Subsidence. Reservoir compaction also can result in surface-subsidence issues, especially when subsidence is significant or where there is little ability to tolerate surface movement because of topside equipment or environmental restrictions. Several analytical and numerical methods are used to predict subsidence. Detailed numerical-modeling methods were developed to quantify surface subsidence. Fully integrated FEA models that honor reservoir layering and seismic interpretations of various subsurface formations are used to estimate subsidence.