Reconciling subsurface uncertainty with the appropriate well design using the Mechanical Earth Model (MEM) approach

Abstract

Abstract The involvement of the well engineer (WE) competency from the earliest phases of exploration is reaping economic benefit for major capital projects (single wells costing at least $25,000,000 US). As the geological and geophysical modeling work of the Explorationists matures and the subsurface picture becomes clearer, the appropriate well design flexibility is being achieved using Mechanical Earth Model (MEM) technology. This approach to well planning is particularly beneficial for deepwater exploration. For deepwater subsalt plays, the capital outlays are immense, with single wells costing up to $100,000,000 US. For these projects, MEM technology has been found to be extremely valuable, particularly in characterizing drilling risks associated with well exit points below salt where in-situ stress perturbations can occur. These stress perturbations are being reliably mapped using MEM techniques and the consequences to borehole stability addressed. An example of principal stress rotation gleaned from a deepwater MEM and the consequences to borehole stability is summarized in this paper. Introduction Formation strength and in-situ stress are the key components of the Mechanical Earth Model (MEM) critical to well engineering design. Figure 1 shows rock stress and strength values (yellow shading) that are calibrated to formation properties and offset well performance criteria to the left. For instance, formation layer rock physics or elastic moduli values must be in agreement with the lithology types coming from the geological model and the impact on formation strength and stress character reconciled. Drilling performance for offset wells, such as rate of penetration (ROP) modeling, is also used to constrain the formation strength estimate. Production history is a general term that includes hydraulic fracture and sand production performance in the reservoir. Core data is used to constrain the static elastic moduli values estimated from acoustic data sources. Figure 1. Mechanical Earth model (MEM) flowchart with key components formation strength and in situ stress. Figure 2 displays rock physics impact on formation strength and elastic moduli with changing porosity at a single depth. As porosity decreases, the corresponding rock modulus and strength increases. When formation layer modulus is known, and the distribution of these layers is also known throughout the MEM volume, the corresponding stress distribution through this volume can be reliably estimated using numerical modeling techniques. Below it will be shown that in tectonically active areas, the high modulus formation beds are propagating relatively higher in-situ stress across the geological structure.