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The basement reservoir is distributed predominantly in Northern Slope Belt of Bongor basin in Chad and adopts openhole completion. Layered well testing is performed in basement reservoir to acquire fluid property, find water-oil interface, confirm oil production rate, etc. Sometimes more than two layers will be tested one by one from the bottom up in the openhole interval. Cementing plug technology is often applied to seal the tested zone and the cement plug also makes a new bottom hole for the MFE testing string to reduce the tail pipe length. But the fractures of basement openhole interval are well developed, so cement slurry leakage often occurs and makes it difficult to control the top of the cement plug, which makes a low success rate while cementing plug. Repeatedly cementing plug operations for one layer makes testing period and cost increased. In order to solve the aforementioned problems and through investigating and analyzing the basement reservoir lithology, a new type of tool, drillable openhole bridge plug, is developed after many laboratory experiments. It is combined with cementing plug to seal the tested zone and successfully used in six (6) wells, simplifying the plugging technology for basement openhole well, raising the success rate of plugging operation and making the plugging position more accurate.

For a well performance drilling fluid, when it operates in deep wells under high temperature, the most important property required is the thermal stability. The drilling fluid properties under high temperature can be controlled by proper selection of viscosity stabilizer, which can capture oxygen to protect polymer agent in the drilling fluid. In this paper a viscosity stabilizer PB-854 is described, which was synthesized by 4-phenoxybutyl bromide, paraformaldehyde, and phloroglucinol using etherification method and condensation reaction. We studied the effect of catalyst dosage, temperature, time, and stirring rate on the synthetic yield. Under this condition: molar ratio of 2-tert-Butylphenol, paraformaldehyde and phloroglucinol of 2:1:2.5, reacting temperature of 100 °C, stirring rate of 100 r min−1, and mass content of catalyst of 15 %, char yield of 5-bromine-3-tert-butyl salicylaldehyde reached 86 %. Under this condition: molar ratio of 5-bromine-3-tert-butyl salicylaldehyde and phloroglucinol of 4, reacting temperature of 60 °C, reacting time of 30 min, volume content of sulphuric acid of 80 %, char yield of the target product viscosity stabilizer PB-854 is 86%. Finally, in this paper, infrared spectroscopy is adopted to analyse the structure of the synthetic product PB-854.The improvement in the stability of drilling fluid was further shown after adding the viscosity stabilizer in the common polymer drilling fluid under high temperature conditions of 120 °C ~ 180 °C. The results show significant change in terms of fluid stability in the presence of this new stabilizer as it provides better stability.

Under the circumstance that international safety and environmental standards being more and more stringent, drilling engineering is facing unprecedented challenges, the extensive traditional process flow is no longer accepted, the new safe and environment-friendly process is more suitable to the healthy development of the industry. In 2015, CNPCIC adopted environment-friendly drilling technology for the first time in the Chad region, ensured the safety of well control, at the same time increased the environmental protection measure, reduced the risk of environmental pollution what obtain the ratification from local government. This technology carries out recovery and disposal of crude oil, cuttings and mud without falling on the ground. The final products are used in road and well site construction, which realizes the reutilization of drilling waste, reduces the operating cost, and provides a strong technical support for cost-cutting and performance-increase of drilling engineering under low oil price.