Prediction of Unconfined Compressive Strength of Microfine Cement Injected Sands Using Fuzzy Logic Method

In this study, unconfined compressive strength values of sand soil injected with microfine cement were predicted using fuzzy logic method. Mamdani and Sugeno methods were applied in the fuzzy logic models. In addition, a regression analysis was carried out in order to compare these two methods. In the models, water/cement ratio and injection pressure were the input variables, and unconfined compressive strength was the output variable. The dataset includes 427 samples, which were experimentally injected with microfine cement. Predictions for unconfined compressive strength were obtained by creating membership functions and rule base for each input (predictive) parameter in fuzzy logic models. The coefficient of determination (R2) and Mean Square Error (MSE) were used as criteria for evaluating the performance of the developed models. The results suggested that the three applied models (i.e. Mamdani, Sugeno and regression) provided statistically significant results, and these methods could be used in the future prediction-based studies. The results showed that Sugeno model provided the best performance for predicting unconfined compressive strength. It was followed by Mamdani and Regression models, respectively. This study has suggested that the fuzzy logic method can be an alternative to the regression method which traditionally has been used in prediction process.

An Enhanced Microfine Cement Design for Special Squeeze Applications

A new and enhanced microfine cement system is presented in this paper which can be used in challenging cement squeeze applications. There are numerous cement squeeze jobs conducted during workover operations every year within the State of Kuwait to prevent water influx. A very common challenge encountered during these applications is either low or no injectivity scenarios. Conventional cement slurries at 15.8-lb/gal density have more often than not resulted in failures while performing post job positive and negative pressure tests, even when the pressure tests are repeated multiple times. These failures can often be attributed to the fact that effective squeezing is not possible due to the larger cement particle size across a limited number of perforations due to early bridging of the cement. Similarly, conventional microfine cement systems which have also been used in these applications have had only limited success. To overcome these challenges, an improved and enhanced microfine cement design has been developed which is able to obtain higher compressive strengths at lower slurry densities (e.g. 12.5 to 13.0 lb/gal) versus the 15.8-lb/gal conventional slurries. This microfine cement design can be further modified to be used in high, low, and zero injectivity scenarios. It possesses several unique features including thixotropic, expansion, anti-gas migration, and strength retrogression properties. Initial field trials of the system have been very successful. The application of conventional microfine slurry systems in low injectivity scenarios is relatively common in the industry; however the enhanced microfine slurry design can be utilized in a variety of injectivity scenarios, or even in loss situations across perforations, casing leaks, or across the casing shoe. The new microfine cement slurry design has the potential of avoiding multiple squeeze jobs by achieving successful positive and negative pressure test results in a minimum number of attempts.

Production Casing Regains Integrity by Repairing Shallow Leak with Microfine Cement

While carrying out planned workover activities, production casing had to be tested to verify integrity. A shallow leak was observed across the production casing at about 1000 ft MD from surface. External and/or internal casing corrosion especially across relatively old wells, could lead to such inconsistency, moreover that major challenge to address and react such surprises online while operation meanwhile nothing to help as a pre-planning studies. Further production objectives require casing integrity as per ESP design to protect shallow aquifer sources. Numerous attempts to repair the casing leak by performing remedial squeeze operations utilizing conventional cement slurry designs proved unsuccessful. Other ideas were considered such as deploying a scab liner to cover the damaged casing section, however this was discounted as it would introduce undesirable borehole restrictions; as well as extra 6 days for running, cementing and clean-out. A novel engineered approach with Microfine cement slurry design squeezed into the damaged zone successfully regained casing and well integrity, while maintaining full borehole access through the production casing, thus saving rig time and tangible cost. Well drilling operations should be carefully designed and executed targeting the integrity assurance by providing the right casing metallurgy and good cementation for proper zonal isolation behind individual casing strings to mitigate external casing corrosion and act as the first line of defense against corrosion and any potential leakage or cross flow among different formations across the life span of the well. Well completions should be installed / tested considering the protection of inner production casing / liner from getting in prolonged contact with wellbore fluids, to avoid excessive internal corrosion and achieve reservoir fluid containment across the life of the well. A proactive approach been raised to acquire production casing / liners corrosion logs across workover activities especially regarding old wells to enable the mapping and interpretation of casing wall thickness / corrosion progress along the well life. That approach could aid in predicting the condition of production strings before commencing planned workover activity; and thus justify the availability of back-up repair plan to maintain rig operations and avoid any unscheduled operational surprises and possible strategic production rate defer.