Managing Sanding Risk in Sandstone Reservoir Through a New Constitutive Model

Sandstone reservoir failure during hydrocarbon production can cause negative impact on the oil/gas field development economics. Loss of integrity and hydrocarbon leakage due to downhole or surface erosion can decrease the risk of operational safety. Therefore, a proper understanding of the best formulation to manage and find the balance between productivity and sand risk is very important. Making decisions for the best and most economical completion design needs a full and proper sanding risk analysis driven by geomechanics modeling. The accuracy of modeling the reservoir rock mechanical behavior and the failure analysis depends on the selection of the constitutive model (failure criteria) specially to understand the failure and post failure mechanisms. Thus, an appropriate constitutive model/criterion is required as most of the current model/criteria are not developed for a weak rock material honoring the non-linearity and post failure (softening) process. Therefore, a new and novel elasto-plastic constitutive model for sandstone rock has been investigated and developed. The effort started with a sequence of triaxial tests at different confining pressures on core samples. Different types of rock have been tested during the developing and validation of the constitutive model. Comparison with other existing failure criteria was also performed. As the results, the newly developed constitutive model is better honoring the full spectrum of elasto-plastic rock mechanical behavior (softening and post-failure) which is important for oil and gas applications, specifically for sand production and drilling i.e. failure stabilization due to stress relief. The formulation and process are demonstrated with a case study for an old gas field, where a few gas wells have been shut-in due to severe sand production. The sand production predictive models have been validated with downhole pressure. The wells have been side-tracked and recompleted using the new sand failure prediction, using the new formulation resulted in restoring sand-free production at former rates. The novelty of this study would be in finding the right formula to best design the predictive model and to avoid any sand production when using the newly developed constitutive model.

Introduction to this special section: Geomechanics

The term “geomechanics” means different things to different people. We assert that in the petroleum industry the broad consensus for a definition would probably be something like this: Geomechanics is the discipline that investigates rock mechanical behavior in the subsurface (i.e., at the wellbore wall, the overburden, cap rock, and/or the reservoir) under present-day in-situ stress and pore pressure conditions or those changed through human activity/intervention (e.g., production, injection, stimulation) during the life of a well.

Pore Properties as Indicators of Deterioration Mechanisms in Slightly Weathered Tuffs

The mineralogy and chemistry of tuff rocks are variable and heterogeneous due to volcanic activity and hydrothermal alteration, in addition to weathering, which makes it difficult to explain the deterioration mechanisms of the weathered rocks based merely on mineralogical and chemical parameters. Studies of tuff weathering indicate that subtle weathering can modify pore structure and subsequently affect the rock mechanical behavior, suggesting that quantitative pore structure parameters are important indicators of the tuff deterioration mechanism. We identified the pore size distribution of pore bodies and pore throats of both slightly weathered tuffs and fresh tuffs using nuclear magnetic resonance technique and mercury intrusion porosimetry. Meso-level uniaxial compression tests were conducted on the tuff samples under a stereomicroscope using MTI-LMs (miniature tensile instrument-light microscopes) to obtain information regarding crack propagation and the deformation process. A comparison of pore properties of slightly weathered tuffs and fresh tuffs indicates that the introduction of additional mesopores (10–50 nm) and pore throat expansion occurs during weathering. The result of mechanical experiments reveal that alteration of the pore structure influences the tuff failure mode. Slightly weathered tuffs show shear failure as cracks initiate in the altered minerals or matrix, while the fresh tuffs exhibit tensile failure as cracks initiate in the intact and fresh minerals and matrix. Based on the results presented here, it is considerable to regard tuff pore properties as potential indicators of the micro-mechanism of substantial macro-deterioration due to weathering.