2021 SPE Drilling & Completion 1 Investigation of the Internal Blowout Accident Involving Overpressured Reservoirs: Case of CI-11 Well, Southern Tunisia Chaouki Khalfi 1 * and Riadh Ahmadi 1 1 National Engineering School of Sfax (ENIS): Laboratory of Water, Energy and Environment (LR3E) Summary This study consists of an assessment of the ecological accident implicating the Continental Intercalaire-11 (CI-11) water well located in Jemna oasis, southern Tunisia. The CI-11 ecological accident manifested in 2014 with a local increase of the complex terminal (CT) shallow water table salinity and temperature. Then, this phenomenon started to spread over the region of Jemna, progressively implicating farther wells. The frst investigation task consisted of logging the CI-11 well. The results revealed an impairment of the casing and cement of a huge part of the 9⅝ in. production casing. Historical production records show that the problems seem to have started in 1996 when a sudden production loss rate occurred. These defciencies led to the CI mass-water fowing behind the casing from the CI to the CT aqui- fers. This ecological accident is technically called internal blowout, where water fows from the overpressurized CI groundwater to the shallower CT groundwater. Indeed, the upward CI hot-water fow dissolved salts from the encountered evaporite-rich formations of the Lower Senonian series, which complicated the ecological consequences of the accident. From the frst signs of serious water degradation in 2014 through the end of 2018, several attempts have been made to regain control of annular upward water fow. However, the fnal CT groundwater parameters indicate that the problem is not properly fxed and communication between the two involved aquifers still persists. This accident is similar to the OKN-32 case that occurred in the Berkaoui oil feld, southern Algeria, in 1986, and included the same CI and CT aquifers. Furthermore, many witnesses claim that other accidental communications are probably occurring in numerous deep-drilled wells in this region. Concludingly, Jemna CI-11, Berkaoui OKN-32, and probably many other similar accident cases could be developing regional ecolog- ical disasters by massive water resource losses. The actual situation is far from being under control and the water contamination risk re- mains very high. In both accidents, the cement bond failure and the choice of the casing point are the main causes of the internal blowout. Therefore, we recommend (1) a regional investigation and risk assessment plan that might ofer better tools to predict and detect earlier wellbore isolation issues and (2) special attention to the cement bond settlement, evaluation, and preventative logging for existing wells to ensure efective sealing between the two vulnerable water table resources. Besides, in the CI-11 well accident, the recovery program was not efcient and there was no clear action plan. This increased the risk of action failure or time waste to regain control of the well. Consequently, we suggest preparing a clear and efcient action plan for such accidents to reduce the ecological consequences. This re- quires further technical detailed study of drilling operations and establishment of a suitable equipment/action plan to handle blowout and annular production accidents. Introduction Current drilling techniques have shown remarkable technological improvements that allow reaching target reservoirs (i.e., hydrocarbons, water, or geothermal) in timely, environmentally friendly, and safe conditions. Rigorous considerations are taken for the preparation of the well profle with respect to the geological prognosis. Behind the technical success of production from targeted reservoirs, the drilling program ensures proper isolation and efective sealing between superposed natural resources. This guarantees water resources protection, rational production, and sustainable development. In North Africa, a major transboundary aquifer system, known as the North-Western Sahara Aquifer System (NWSAS), is shared between Tunisia, Algeria, and Libya. This aquifer system hosts huge reserves of nonrenewable water in a region characterized by an arid climate and scarcity of surface water resources, providing vital needs for autochthonous populations. Therefore, water wells satisfy all the regional water needs. Besides, this zone represents Africa’s biggest oil-producing province; thousands of oil wells cross both NWSAS aquifers, causing a serious water contamination risk. In 2014, in Jemna oasis, located in Kébili governorate, a sudden geochemical degradation and temperature increase occurred on the CT freshwater well. This ecological accident is likely to be related to the CI-11 deepwater well located in the neighborhood. The accident consists of a remarkable increase in salinity and temperature that progressively infuenced wider surfaces over time. The consequence of large water contamination is a real threat at a regional scale. In fact, the Kébili region is characterized by an important agricultural poten- tial. This agricultural activity consists of high economic value products such as dates, with a regional contribution of approximately 58% to the national production. Therefore, besides the environmental impact, the contamination of the water resources in this region engenders local direct socio-economic consequences and subsequent national economic damage. Therefore, this work aims to recover the available data to analyze and investigate the origins of the CI-11 ecological accident. This study focuses on a scientifc diagnosis of the problem, identifcation of the causes, and the assessment of the environmental impact of the CI-11 drilled well on the groundwater resources of the region. *Corresponding author; email: chawki.khalfi@gmail.com Copyright © 2021 Society of Petroleum Engineers Original SPE manuscript received for review 4 March 2021. Revised manuscript received for review 4May 2021. Paper (SPE 208598) peer approved 13 September 2021.