How do you evaluate and mitigate the risks of wellbore instability and formation damage?

1 Causes of wellbore instability

Wellbore instability, referring to the failure of the wellbore wall due to the interaction between the rock and the drilling fluid, can manifest as borehole collapse, caving, fracturing, or sand production. The primary causes of this instability are mechanical stresses, such as in situ stresses, pore pressure, and drilling-induced stresses; chemical effects, like changes in pH, salinity, or ion exchange; and thermal effects from temperature differences between the drilling fluid and the formation. These effects can weaken the rock matrix, dissolve cementation, swell clays, induce thermal stresses or expansion/contraction of the rock, and cause cracking or spalling.

2 Methods of wellbore stability analysis

Wellbore stability analysis is a process of predicting and preventing wellbore instability through the use of various data sources, models, and tools. Geomechanical modeling is employed to construct a 3D model of the stress, strain, and failure behavior of the rock around the wellbore. This helps to optimize the well trajectory, casing design, and mud weight. Wellbore imaging uses logging or measurement while drilling (MWD) tools to acquire high-resolution images of the wellbore wall in order to identify the type, location, and severity of wellbore instability and calibrate the geomechanical model. Real-time monitoring is also used through sensors, gauges, or telemetry systems to measure and transmit various parameters related to wellbore stability such as pressure, temperature, torque, drag, vibration, or cuttings in order to detect and respond to wellbore instability events and adjust drilling parameters accordingly.

3 Best practices for wellbore stability management

Wellbore stability management requires the application of wellbore stability analysis results to design and implement the optimal drilling and completion strategy. Best practices include selecting a drilling fluid compatible with the formation and providing adequate hydrostatic and lubricating properties, as well as minimizing fluid invasion, filtrate loss, or solids accumulation. Additionally, drilling practices should be tailored to the geomechanical conditions and wellbore stability objectives, including proper hole cleaning, bit selection, drilling speed, weight on bit, and circulation rate. Completion techniques should preserve or enhance the wellbore stability and formation integrity, such as appropriate casing, cementing, perforating, gravel packing, fracturing, or acidizing methods. Furthermore, production or injection's potential impact on wellbore stability should also be considered.

4 Causes of formation damage

Formation damage refers to the impairment of the reservoir permeability or porosity due to the interaction between the formation and the drilling or completion fluids, resulting in reduced well performance, increased operational costs, or premature abandonment. Causes of formation damage can include mechanical effects, such as plugging, bridging, or erosion of the pore throats or fractures caused by solids, fines, mud cake, cuttings, sand, or scale; chemical effects like precipitation, dissolution, or adsorption of minerals or organic matter due to incompatibility with the formation fluid or rock; and biological effects like growth, metabolism, or death of microorganisms due to the introduction of nutrients, oxygen, or contaminants into the formation. These biological effects can lead to biofouling, biocorrosion, or souring.

5 Methods of formation damage evaluation

Formation damage evaluation is a process of quantifying and characterizing the extent and nature of formation damage through various data sources, models, and tools. Core analysis, well testing, and numerical simulation are some of the methods used to evaluate formation damage. Core analysis involves laboratory tests to measure the permeability, porosity, or fluid saturation before and after exposure to drilling or completion fluids. Well testing involves field tests to measure pressure, flow rate, or production data before and after drilling or completion operations. Numerical simulation uses mathematical models to simulate fluid flow and transport phenomena in the reservoir and near-wellbore region, helping to predict the formation damage potential, distribution, and evolution.

6 Best practices for formation damage prevention

Formation damage prevention is the application of the formation damage evaluation results to design and implement the optimal drilling and completion strategy. To achieve this, fluid design should be based on the formation characteristics and fluid compatibility tests, with an aim to minimize fluid invasion, filtrate loss, or solids content that can cause mechanical damage, as well as optimize fluid chemistry, rheology, or additives to prevent chemical or biological damage. Additionally, proper handling, storage, mixing, conditioning, or recycling of the drilling or completion fluids should be ensured to guarantee fluid quality, stability, or performance and avoid contamination, degradation, or dilution. Finally, appropriate techniques such as acidizing, solvent flushing, water jetting, or hydraulic fracturing should be used to restore or improve the formation permeability or porosity after drilling or completion operations.

7 Here’s what else to consider

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