Use of Obturating Pills Combined with Low Concentration Acid Pills to Overcome Differential Stuck Challenges in Highly Deviated Wells

In 2020, A Major Offshore Operating Company in UAE faced a high differential pressure stuck event. This took place, during the execution of formation evaluation with a conveyed pipe sampling BHA. It is well known that after a differentially stuck pipe event happens, the success ratio will be time dependent (i.e. the less time a pipe remains stuck, the more chances there will be for it to become released) and the chances of releasing the BHA are always limited to the logging tools tensile limitations. One of the most common and successful methods to release differentially stuck BHA's specifically in limestone formations is by pumping and soaking acid pills. However, under such a high differential pressure environment, the use of acid may induce losses in the so called "thief zones", causing worse problems. The standard release procedure started by working the string within the tensile limits, followed by pumping acid pills (using the available volume mobilized before spudding each drilling section). During the first acid pills pumped to attempt to release the stuck pipe, loss volume pump rates, acid pill position(s) and coverage in the annulus was assessed and evaluated. Based on the results it was observed that the thief zone was in direct contact with the Pipe Conveyed circulation port, at the latching assembly. This then created a situation whereby, the acid pills were lost immediately after the acid came into contact with the formation. Utilizing low acid concentration pills also had the same effect. The solution was to pump an Obturating pill made-up of a weighting agent as a spacer behind the high volume, low concentration acid pill with crosslinking divergent chemicals, pumped at a high flow rate. This solution reduced the acid losses across the thief zone and once the Obturating pill came into contact with the loss zone, it facilitated the seal and cured the losses, by allowing the acid to move up the annulus until it reached the differentially stuck point and soaked across the problematic area. Once the acid pill was successfully placed at the required location using this method, the string was worked and successfully released. This combination of low concentration acid along with crosslinking divergent chemicals coupled with the Obturating pill behind the acid helped a major offshore operating company to pump the acid pill under partial losses only, to regain full circulation after only 10 bbl. of Obturating pill had entered inside the annulus. It allowed the acid pill to react across the stuck point, while keeping the string under low torque and compression. The operator was able to release the sampling BHA and bring it back to surface avoiding a lost in hole cost of around 4 MM$. The Obturating pill combined with low concentration CDC acid pill is proven to be a successful method in drilling operations when trying to release differentially stuck BHA's within a wellbore.

A Unified Parameter to Represent Reservoir Heterogeneity

We aim to find a universal method and/or parameter to quantify impact of overall heterogeneity on waterflood performance. For this purpose, we combined the Lorenz coefficient, horizontal permeability to vertical permeability ratio, and thief zone permeability to average permeability ratio, with a radar chart. The area of the radar chart serves as a single parameter to rank reservoirs according to heterogeneity, and correlates to waterflood performance. The parameters investigated are vertical and horizontal permeability. Average porosity, initial water saturation, and initial diagonal pressure ratio are kept constant. Computer based experiments are used over the course of this entire research. We conducted permeability studies that demonstrate the effects of thief zones and crossflow. After normalizing these parameters into a number between 0 and 1, we then plot them on a radar chart. A reservoir's overall degree of heterogeneity can be inferred using the radar chart area procedure discussed in this study. In general, our simulations illustrate that the larger the radar chart area, the more heterogenous the reservoir is, which in turn yields higher water cut trends and lower recovery factors. Computer simulations done during this study also show that the higher the Lorenz coefficient, the higher the probability of a thief zone to exist. Simulations done to study crossflow also show certain trends with respect to under tonguing and radar chart area.

Thief-zone plugging mechanism and application of calcite particles in fractured formations

Lost circulation in fractured strata during drilling incurs additional costs and leads to difficulties in promoting drilling safety and efficiency. Plugging-while-drilling is a feasible means to address lost circulation in fractured formations. The particle size distribution (PSD) of plugging particles is determined empirically; therefore, it is often not matched with the fracture sealing requirement. This study investigates the lost circulation mechanism of fractured strata and identifies a calcite particle-based material as the preferred plugging agent. The plugging mechanism and the design of PSD and particle concentration are demonstrated. Based on the concentration, a plugging-while-drilling technique was developed for fractured strata. The results show that calcite particles tend to form the filling layer at the fracture inlet, which cuts off the leakage of the drilling fluid into the fracture, eliminating the drilling fluid pressure applied on the fracture surface. Thus, a stable sealing for the fractured formation is achieved, and the pressure-bearing capacity of the borehole wall is increased. The result also reveals the optimal mixing and concentration models for calcite particles with various diameters. The plugging technique based on calcite particles for fractured formations is applied in a field experiment. The results confirm that the technique can improve the chance of lost circulation prevention and thief-zone plugging in fractured strata and remarkably reduce both the event quantity of lost circulation and the volume of circulation loss. The findings of this research lay a theoretical basis to address lost circulation in fractured formations and, thus, have important practical significance.

A Novel Solution for Severe Loss Prevention While Drilling Deep Wells

The loss of circulation is a big problem in drilling operations. This problem is costly, time-consuming and may lead to a well control situation. Much research has investigated the effectiveness of using different chemicals as lost circulation material (LCM) to stop mud and cement slurry losses. However, there remain many limitations for using such LCM types, especially when it comes to field applications. This paper presents a new high strength lost circulation material (HSLCM) that could effectively be used for managing severe lost circulation cases. The HSLCM could easily be pumped into the thief zone where it forms a gel that solidifies after a setting time to provide sealing between the wellbore and the thief zone. With this technique, the material stops the circulation losses, and hence enhances the well bore stability by reducing the well bore stresses. The HSLCM has a high compressive strength and it has a high acid solubility of around 96%. Because the HSLCM has high tolerance towards contamination, it can be utilized with water-based mud or invert emulsion-drilling fluids, hence providing a wide window of applications with the drilling fluids. In this study, laboratory experiments were conducted to evaluate the rheology, thickening time, compressive strength, and acid solubility of the HSLCM. The results showed good performance for the HSLCM as LCM. In addition, a case field study is presented which shows a successful field treatment for severe losses.

Evaluation and Utilization of Nano-Micron Polymer Plug for Heterogeneous Carbonate Reservoir with Thief Zones

Thief zones are highly permeable zones in oil reservoirs which affect the performance greatly during water flooding. The most effective way to prevent the invalid circle of thief layer is the injection of high-concentration polymer slugs. In this paper, the plugging effect of nano-micron polymer flooding on the thief zone is studied by experimental and theoretical analysis. The changes of water content and flow resistance were analyzed under different conditions. The result shows that the model presented here has good agreement with the experimental results. The displacement effect is the best when the thief zone is located on the upper part of the reservoir. And the water content will decrease with the increase of flow resistance after nano-micron polymer injection. Besides, the higher the polymer concentration, the more obvious the decrease of water content, and more effective the plugging. This study has provided a quick and reasonable guide in the later adjustment of water flooding development of carbonate reservoirs with thief layers.

Impacts of pore size distribution on gas injection in intraformational water zones in oil sands reservoirs

Intraformational water zones are widely reported in Canadian oil sands fields. In order to pressurize a thief zone, one of the initiatives is to inject gas. However, the evaluation of gas injectivity based on a pore size distribution is still a big challenge. This study provides a multi-scale approach to study the effect of a pore size distribution on gas injectivity in intraformational water zones. The results indicate the gas effective permeability increases in a less complex and more discrete pore network. The enhancement of gas effective permeability with increased gas saturation weakens with higher complexity and lower discreteness of a pore network. A less complex and more discrete pore network better benefits the gas injectivity index.

Identification of the Thief Zone Using a Support Vector Machine Method

Waterflooding is less effective at expanding reservoir production due to interwell thief zones. The thief zones may form during high water cut periods in the case of interconnected injectors and producers or lead to a total loss of injector fluid. We propose to identify the thief zone by using a support vector machine method. Considering the geological factors and development factors of the formation of the thief zone, the signal-to-noise ratio and correlation analysis method were used to select the relevant evaluation indices of the thief zone. The selected evaluation indices of the thief zone were taken as the input of the support vector machine model, and the corresponding recognition results of the thief zone were taken as the output of the support vector machine model. Through the training and learning of sample sets, the response relationship between thief zone and evaluation indices was determined. This method was used to identify 82 well groups in M oilfield, and the identification results were verified by a tracer monitoring method. The total identification accuracy was 89.02%, the positive sample identification accuracy was 92%, and the negative sample identification accuracy was 84.375%. The identification method easily obtains data, is easy to operate, has high identification accuracy, and can provide certain reference value for the formulation of profile control and water shutoff schemes in high water cut periods of oil reservoirs.