Application of Downhole Video Technology to Multilateral Well Completions (1998)

Abstract There are two economically useful applications of downhole video technology during multilateral well drilling and completion operations. The first is the quick diagnosis of installation difficulties during the junction installation procedures and the second is verification of junction integrity after the completion of the junction. Included in the latter is the application of downhole video to evaluate the status after certain intermediate steps during the junction installation procedures themselves. This inspection technology has already been used to decrease the risk involved with multilateral technology through its application during the testing of multiIatera1 completion systems, by evaluating the results of milling operations, and in verifying junction integrity. Introduction There are two steps to utilizing downhole video technology in the multilateral completions. The first step is to establish an optically clear medium in the area of interest. The actual deployment of the video camera (with light source and data transmission system) is only the second step. Without the first step. the second is a waste of time and resources. Turbidity. Turbidity is the reduction of transparency due to the effect of particles. It is an optical property of the liquid. Clarity or clearness is the lack of turbidity. There are turbidity measurement devices that measure transmission of light straight through a liquid, those that measure the scattering of light a various angles to the source beam, and those that combine these two methods. However, turbidity is, only in part, the effect of suspended particles on light passing through a liquid. Selective absorption of various frequencies of light have an effect on the turbidity "measurement". In 1974, the National Oceanic and Atmospheric Administration recommended, after due deliberation, to call turbidity a "non-technical appearance descriptor."' What this means is that turbidity is a phenomenon that involves human perception. Turbidity is therefore, not absolute. It is a hazy concept. If you want to maintain levels of turbidity of a known liquid from day to day, then you must first agree upon how you will measure, what you will measure, and use a similar measurement technique each time. A good rule of thumb is that downhole video is most useful in liquids that have a turbidity of less than I I NTU's (Nephelometric Turbidity Units) measured by one of the devices that includes both transmission and scattering methods in the measurement. However, since the video cameras are working almost entirely in the visible light range (Fig. 1). The best way to measure turbidity is by visually looking through the fluid yourself, sampled at the wellhead and not at the pump or holding tank.' The turbidity of the fluid can change considerably going through pumps and lines that are less than clean. If you can see through it for 10 inches, the video camera can see through it for 10 inches. The kelly hose should be avoided. It is can be a continuing source of particulate matter that reduces visibility. If a brine solution is used, it should be premixed and placed in a holding tank at least one day previous to being filtered and pumped downhole.