The physico-chemical properties and behaviors of 5 different mud samples were investigated to evaluate their performances for use in drilling operation, “Tønder geothermal drilling” as case study.
The project focuses more on the laboratory experiments carried out on the mud samples at different conditions of temperatures and pressures, aging times, quantities of additives e.t.c. Two different water sources,“fresh tap water” and “sea water” from Fanø beach were used as the continuous phase for the experiments. Different additives were employed in evaluating the mud performances. Duo Tec NS, Bentonite and CMC were viscosifiers used, MI PAC Elv, Bentonite, CMC and Starch were the fluid loss additives used, Sodium hexametaphosphate (SHMP) and sodium pyrophosphate decahydrate (SPP) were the polyphosphates applied for thinning effects. Other materials provided include glycol and potassium chloride for inhibition of clay/shale, soda ash and barite for reducing calcium and for weighting the mud respectively. Apart from experiments carried out on the mud directly, experiments were also run on the mud filtrates to determine their properties and how they affect or deter the success of the drilling operation with regards to their interaction with the formation.
Results from the experimental works showed that all mud samples provided had a good swellability and could be used for coating of the side walls of well bore. For the viscosity reduction test, though both polyphosphates presented did their jobs of reducing viscosity, sodium pyrophosphate decahydrate showed more superior thinning effects than sodium hexametaphosphate. Samples 4 and 5 were seen to have the least sand contents both in volume and weight percents, while sample 2 contained the most sand.
Regarding the test with viscosifiers, both CMC and Duo Tec NS have proven they can be used for increasing viscosity effectively, however, Duo Tec showed more superior qualities in terms of mud rheology. It has a higher yield point and and gel strength than CMC which is a property well desired of a viscosifier. In the swell inhibitive test, inhibition was mainly due to the potassium ions present in KCl, and that glycol could not effectively inhibit absorption of water except in combination with KCl.
In addition, CMC has been proven to have very good fluid loss properties compared to other additives used, but MI PAC has also been seen to be another fluid loss agent that can competitively give similar results as CMC. Both would be recommended as fluid loss agents, allowing the mud sample to produce filter cakes of desirable properties.
Filtrate analysis showed that the levels of calcium, magnesium and chlorides present in the filtrates were at acceptable levels and further treatment would not be necessary and alkalinity tests showed the ions present in the filtrate in terms of carbonates and bicarbonates using a stoichiometric classification as well as the amounts of acids needed to reach the phenolphthalein and total alkalinity end points.
The effects of aging were inconclusive from the experiments, but generally, it showed that aging reduced rheological properties. Temperature effects also showed that increasing temperature from ambience to 74oC generally decreased rheological properties. Filter cake was formed with pressure increase, and the concepts of balanced, over balanced and underbalanced holes were verified from the balance between hydrostatic and formation pressures.