Logging Shale Barrier before Well Abandonment

When a well has served its purpose is it important to abandon it in a state that prevents leakage of hydrocarbons. The regulatory requirement for the Norwegian Continental Shelf is the existence of at least three barriers when the well is abandoned. With today's technology, where 20-60 days are spent on plugging and abandonment (P&A) of a well, the cost was estimated in 2014 to be almost 900 billion NOK for the wells on the Norwegian Continental Shelf. And the Norwegian Government would have to cover 78 %. By using the shale surrounding the production casing as a barrier it is possible to leave the production casing in place and save considerable time (and money). One of the challenges related to use the shale as a barrier relates to the verification of such a barrier around the casing. Today, acoustic methods are used to measure the annulus, but the interpretation of these measurements are difficult and often give an unclear image. In this project, we aimed at getting a better understanding of what is affecting the acoustic measurements in order to improve the methods and interpretations. This was done through advanced laboratory experiments with downscaled boreholes in shales. In these experiments an acoustic tool developed in this project is used, and which corresponds to a downscaled version of the tools used in the field today. Numerical simulations give insights in the acoustic logging methods used today and together with analysis experimental data have we improve the understanding and found possible improvements of the measuring methods used today in the industry. The project has shown that the ultrasonic methods used in the field may be downscaled and used in holes less than 20 mm in diameter. This has given us the possibility to perform shale barrier experiments at realistic stress and measure the effect on the acoustic logs as the material surrounding the casing changes. Several experiments have been performed in the project where the shale has created a barrier at realistic field conditions. Continuous ultrasonic measurements through the casing shows changes as a function of time and barrier quality and permeability. The gives new and unique insights of the connection between the measurements and the quality of the shale barrier. These results will put us in better position to understand the measurements done in the field and to choose the right strategy in order to reduce costs when the time comes for abandoning the oil production on the Norwegian Continental Shelf.

The project has contributed to the general understanding of cement bond log response to fluid, heavy fluid and solids in the annulus. Special contribution has been to the understanding on how a creeping shale/potential shale barrier may affect the cement bond logs. This has the potential of improving the interpretation of field cement bond logs and make the operator better to identify possible barriers with the cost reduction that may give. A special experimental facility has been developed in this project to perform cement bond logging under field-like conditions. This facility has the potential to be used to study many field-relevant scenarios where the cement bond logs may be difficult to interpret. This includes testing field shales for log response, settled baryte from muds, and top of cement identification. The test facility will be used in a follow up project "Shale Barrier Toolbox" sponsored by RCN and several operators.

This project aims at improving shale-bond logging interpretation. This will be done by performing laboratory experiments on hollow cylinder shale samples with acoustic logging in a cased hole, permeability measurement in the annulus and stress and pressure control of the sample and hole. The acoustic logging tools to be applied in the experiments will be downscaled versions of standard cement bond logging tools, and hence the results obtained from these experiments can directly be compared to cement bond logs run in the field today. Numerical simulations tailored to study logging through casing will be used to design experiments, and to help interpret the measurements. In addition, a systematic feasibility study will be done on the potential for of using non-linear acoustics to quantify fracture densities in shale barriers, and improve seal-quality assessment.