Known as an underbalanced condition, this occurs when, in the wellbore, formation pressure is higher than the hydrostatic pressure and this lead to a well control situation. To overbalance formation pressure, the required hydrostatic pressure is normally provided through an adjustment in drilling fluid density. Hydrostatic pressure loss can occur for a number of reasons;
- ECD loss
- Surface drilling fluid dilution
- Cement density reduction
- Drilling process releasing formation fluids
- Weighting material movement from mud cleaning equipment
- Drilled cuttings or mud weighting materials settling
Since a reduction in the density of mud returns is sometimes happened, most wells are designed to have sufficient overbalance to encounter the small reduction of mud density and this should prevent a kick. However, if there is significant mud weight reduction, an investigation shall be performed to find any root cause and provide any preventive actions.
Lost circulation whole mud (whether to depleted reservoirs or to natural/induced fractures)is one of biggest causes of well kicks. In the wellbore, fluid levels can decrease and this lowers the hydrostatic pressure. Once hydrotatic pressure is less than formation pressure, it will cause a flow from the formation in permeable zones. Figure 1 illustrates loss of fluid level into a weak zone which will lead a well control incident if a wellbore is not filled up on time with correct mud weight.
Figure 1 – Lost Circulation in Drilling Leading to a Well Control Situation
Primary well control is the most important barrier while drilling and completing any wells so it is imperative to ensure that the primary well control is effectively maintained.
When various precautions and procedures have been followed, effective primary well control can be achieved. These procedures can be seen below;
Using a trip sheet (an accurate log), it is possible to maintain tripping both in and out of the well. A trip sheet can help to record the volume of mud that not only enters the well but also that is displaced when tripping. During tripping, the changes in mud volume can be measured using a calibrated trip tank.
For any steel removed, a specific amount of mud is entered into the well when the tripping pipe or drill collars from the hole. To ensure proper well monitoring, tripping may need to be stopped whenever the volume of removed steel significantly outweighs the volume of mud required. After stopping, consideration should also be made towards returning back to bottom in order to condition the mud (and find the cause of the issue). At all times, the drill floor should have the required crossover subs and a full opening safety valve readily available. Continue reading
Well control can be categorized into three main categories which are Primary Well Control, Secondary Well Control and Tertiary Well Control. The details are shown below;
Primary Well Control
Primary Well Control is hydrostatic pressureprovided by drilling fluid more than formation pressure but less than fracture gradient while drilling. If hydrostatic pressure is less than reservoir pressure, reservoir fluid may influx into wellbore. This situation is called “Loss Primary Well Control”. Typically, slightly overbalance of hydrostatic pressure over reservoir pressure is normally desired. The basic of maintaining primary well control is to maintain hydrostatic pressure that is heavy enough to overcome formation pressure but not fracture formations.
Figure 1 – Drilling Fluid
Not only is hydrostatic pressure more than formation pressure, but also hydrostatic pressure must not exceed fracture gradient. If mud in hole is too heavy, it will cause a broken wellbore, that will result in loss circulation problem (partially lost or total lost circulation). When fluid is losing into formation, mud level in well bore will be decreased that will cause reduction in hydrostatic pressure. For the worst case scenario, hydrostatic pressure is less than formation pressure therefore wellbore influx (kick) will enter into wellbore. Continue reading
Wild Well Control is one of the best well control specialist companies in the world. The company not only provide well control and engineering services to the company’s customers, it also provides free technical knowledge to the public such as Wild Well Control Technical Book that you can download it for free. Today, we would like to review one of the most useful presentation which is “Well Control Methods” presentation. After you read the review and you like it, we also provide download link for you at the end. Thank Wild Well Control for great contribution to oil and gas industry.
Well Control Methods by Wild Well Control, Wild Well Control (2017)