Nobuo Morita

A Quick Method To Determine Subsidence, Reservoir Compaction, and In-Situ Stress Induced by Reservoir Depletion

This paper provides a quick method to determine subsidence, compaction, and in-situ stress induced by pore-pressure change. The method is useful for a reservoir whose Youngs modulus is less than 20% or greater than 150% of the Youngs modulus of the surrounding formation (where the conventional uniaxial strain assumption may not hold). In this work, a parameter study was conducted to find groups of parameters controlling the in-situ stress, subsidence, and compaction. These parameter groups were used to analyze the numerical calculation results generated by a three-dimensional (3D), general, nonlinear, finite-element model (FEM). The procedure and a set of figures showing how to calculate the in-situ stress, subsidence, and compaction induced by pore-pressure changes are provided. Example problems are also included to prevent confusion on sign convention and units. This work showed that Geertsmas results, which are based on no modulus contrast between cap and reservoir rocks, should be extended to simulate more closely real reservoirs, which generally have distinct property differences between the cap and reservoir rocks.

Field and laboratory verification of sand-production prediction models

The many completion options available for completing sand-prone formations include (1) inside-casing gravel packing, (2) openhole gravel packing, (3) selective perforation, (4) a vertical or horizontal well with pre-packed liner, (5) a vertical well with screen, (6) sand consolidation, (7) oriented perforations, and (8) fracture packing. Selection and design of these options depend on analysis of formation strength, strength distribution, permeability, permeability distribution, shale content, fines migration, and grain size and distribution. Of these factors, permeability, permeability distribution, grain size, and grain distribution can be measured with reasonable accuracy; however, most oil companies still have difficulty conducting formation-strength and strength-distribution analyses. The two primary reasons for poor analysis are that mechanical logs available from service companies are not reliable or must be calibrated with other methods and that reasonably reliable numerical models for strength analysis are owned exclusively by several companies. This paper addresses what these formation-strength-analysis models predict, the accuracy required for formation-strength analysis, sources of model errors, model limitations, and methods to improve these models. This paper can be used as a guide for engineers who are planning to develop or to acquire these models.

Stress-Intensity Factor and Fracture Cross-Sectional Shape Predictions From a Three-Dimensional Model for Hydraulically Induced Fractures

An efficient three-dimensional (3D) fracture model was developed that can be applied to arbitrary planar cracks with Mode 1 stress singularity at the fracture tip. Surface stress, thermally induced stress, and poroelasticity were included. In addition, the elasticity constants can be varied between elements. A parameter study was conducted with the model to evaluate the stress-intensity factor, the evolving shape of the fracture, and fracture width. The study includes the borehole effect, fracture migration, fracture barriers, fracture cross-sectional shape, fracture front shape and its stress-intensity factor, and evaluation of the validity of some simplified models. Further work on the details of the theory, algorithm, accuracy, and efficiency of the 3D fracture model is in progress.