Compressive wellbore failure is a major contributor to sanding problems and casing collapse in many oil and gas fields. This type of failure occurs when the wellbore pressure exceeds the rock strength, typically in unconsolidated and weakly-cemented rocks or in consolidated rocks where stimulation programs may have removed or weakened the rock cement. The complex interrelationships between the physical factors that act on the wellbore to result in this type of failure makes the dynamic modeling of wellbore stability challenging. Most of the available models for predicting sanding and well collapse use openhole logs with very little success. The dynamic mechanical properties obtained from logs are typically too optimistic and hence require calibration to static measurements on selected core samples. It is, however, important to address issues relating to the representativeness and the relative volumes of investigation of the core data. The associated uncertainties of the core and wireline log measurement scales and conditions must be considered for an effective model.
This paper presents a systematic core-to-log integration technique that enables the static mechanical rock properties measured on rock samples to be used in the effective calibration of dynamic wireline data for mechanical log analysis. Relationships are developed between static and dynamic mechanical rock properties.
Three different field examples are presented that demonstrate that rock strength, optimum drawdown and compressive failure are related to reservoir rock quality and can be predicted by rock typing or flow units in sandstones. This work demonstrates that the flow units concept in combination with the mechanical logs may be used effectively as predictive tools for identifying weak zones that are prone to mechanical collapse and therefore should be avoided during completions. Some of the applications and benefits of the methodology shown in this paper include:
The validated model can be used to determine the maximum drawdown for the onset of sanding.
The combined use of flow units and mechanical properties can help avoid zones that are prone to mechanical collapse.
Unwarranted sand control operations like gravel packing can be avoided, thereby optimizing field productivity and hence profitability.
Wellbore failure resulting in sanding and sand control is a very critical challenge in oil and gas production. It is critical to address this problem because some of the problems associated with sanding are:
high pressure drop due to sand fill in perforations;
erosion of down-hole and surface tubulars resulting in loss of well control;
separation and handling of sand in produced fluid and
subsidence and casing collapse.
The following subsurface conditions may favor wellbore failure:
water break-through in weak to intermediate strength formations;
reservoir pressure depletion in relatively strong formations and
abnormally high lateral tectonics.
One of the methods typically used to prevent excessive sand production in poorly consolidated reservoir rocks is gravel packing. Gravel packing is expensive and can cost up to 1 million dollars for high-pressure offshore wells. It restricts the wellbore and makes for workovers difficult and expensive. It limits production from the wellbore due to high-pressure drawdown.
Another method that is typically used including moderate to high permeability reservoirs (for formation damage remediation and sand control) is frac-pack. This entails the placement of short, wide, highly conductive propped fractures. This method is very effective for removing formation damage when acidizing fails, and for sand control when gravel pack or other sand exclusion methods fail.