There are various pressure related terms that mudlogging geologists encounter during the course of drilling operations. A clear understanding of these terms will not only help understand a drilling situation but also effectively manage it. This blog will discuss some of the most common pressure terms used on oil rigs:
Standpipe Pressure (SPP): SPP is the primary pressure that a Mudlogger has to monitor during the course of oil well drilling. Roughly speaking it is the pressure required to circulate drilling fluid in the hole. An experienced mudlogger with knowledge of SPP behaviour can easily pick poor down-hole conditions (which may lead to drill pipe sticking), or a washed out bit nozzle or a plugged bit nozzle (creating less than optimum bit hydraulic horse power) or a washout in drill pipe ( that may lead to parting of drill string) and thus help avoid drilling complications. There are many factors that effect SPP such as mud density and mud rheology, measured hole depth, hole geometry including string OD and ID and sizes of bit nozzles etc. Knowing these parameters, it is possible to theoretically calculate total pressure losses which should be equal to SPP.
Hydrostatic Pressure: It is the pressure exerted in the hole by the column of mud. Amount of hydrostatic pressure being exerted in the hole depends upon mud weight (density of drilling fluid) and vertical depth of the hole. Using common oil industry units, hydrostatic pressure can be calculated by this simple formula: MW in ppg x 0.052 x TVD. If the MW is 10 ppg and depth is 3000 ft TVD, the hydrostatic pressure would be 1560 psi. hydrostatic pressure plays a crucial role in the safety of drilling operation. Proper hydrostatic pressure prevents borehole from collapsing and avoids getting kick and blow out.
Formation Pressure: Formation pressure is the pressure exerted by formation fluid (oil gas and water) contained in the pore spaces of the rocks. Terms like reservoir pressure and pore pressure are sometimes used interchangeably with formation pressure. When formation pressure is equal to hydrostatic pressure (8.4 to 9.0 ppg) it is called normal formation pressure. However when formation pressure exceeds 9.0 ppg it is called over pressure. And when formation pressure is less than normal pressure, (as seen in some hydrocarbon reservoirs of old fields) it is called subnormal or depleted pressure. When formation pressure exceeds hydrostatic pressure, we get kick (influx into the borehole) an uncontrollable kick may cause blow out (most dreaded accident on an oi rig). Therefore knowledge of formation pressure is required to control it by maintaining slightly higher hydrostatic pressure. A rough idea of formation pressure can be taken from offset wells or from pressure logs during drilling. Accurate formation pressure is known by performing pressure tests on wireline or when doing production test at the end of well TD. LWD logging companies provide pressure testing tools that can tell formation pressures while drilling but they are time consuming and expensive at present technological level. Accurate formation pressure can also be calculated from shut in drill pipe pressure (SIDPP), when the well is shut in after getting the kick.
Surge And Swab Pressures: These pressures come into play when the bit is moving up or down in the hole irrespective to whether pumps are on or off. When the bit moves down it creates extra hydrostatic pressure (surge pressure) below the bit and when the bit is pulled off it has reducing effect on hydrostatic pressure(swab pressure). Surge and swab pressures depend upon many factors such as mud properties, hole geometry and speed of pipe movement. When using high MW close to fracture gradient, surge pressure should be maintained as low as possible in shoe area to avoid fracturing formation and losing mud. On the other hand if we are drilling overpressure zone, swab pressures should be kept at minimum to avoid influx.
Fracture Pressure: Once we drill a section of hole, we case and cement it to avoid getting it collapsed at later stage. After drilling out cement from inside casing and about ten feet of new formation we usually perform Leak Off Test (LOT) to check the formation strength. In order to perform LOT, we close the annuls using BOP and slowly pump mud into the hole to pressurize the formation. The moment pressure stops increasing with same rate is the pressure when formation breaks down or develop fractures. And the pressure at which this happens is called fracture pressure or leak off pressure. Knowledge of fracture pressure is vital to have before drilling out the phase for two reasons: One, because it sets out our limit of maximum MW. By crossing this limit we will fracture the formation and start losing the mud into the formation. Two, if we get a kick and circulate it out we have to maintain bottom hole circulating pressure to less than fracture pressure otherwise will fracture formation at shoe and invite more influx in the borehole, which will be a very difficult situation to be in.For easy understanding, fracture pressure can be converted into equivalent MW by using the formula: Fracture Pressure (psi) / 0.052 / Depth (tvd). Example: if fracture pressure is 1900 psi and shoe depth is 2100 ft tvd. Fracture Pressure MWeq. = 17.4 ppg. So the new section can only be drilled safely by maximum of 17 ppg MW (leaving 0.4 ppg as safety margin). At this point one may ask a question: what if formation pressure in the section is expected to increase to 17 ppg+ requiring roughly 17.5 ppg MW to control it? The answer is we have to stop drilling as soon as we reach near 17 ppg MW, set casing and drill out high pressure zone in the next section which will definitely have higher leak off pressure and will give us more room to increase MW. Another question that can be asked is: Why LOT/FIT is performed just below the shoe? Why not at deeper depth of the section? The reason is that formation just below the shoe is the weakest for that section. With depth formation strength increases as the overburden pressure increases. Formation pressure and fracture pressure from offset wells are dully taken into consideration in deciding various casing shoe depths.
Overburden Pressure: Also known as vertical stress, is the pressure exerted by overlying rock layers (solid rock grains plus the pore fluid) at a given depth. Overburden pressure increases with depth as a result formation become more and more compact and gain strength with depth. Based on the above statement, it can be said that overburden pressure depends upon the height of rock column and density of rock column. Mathematically this can be expressed as: Overburden pressure (S in psi) = 0.433 x formation density (gm/cc) x depth interval (in feet). Data engineers and pressure engineers calculate overburden pressure or vertical stress using wireline or LWD density logs. From vertical stress they estimate the other two stress components on X&Y axes. With complete knowledge of stress distribution on X, Y and Z axes, they calculate fracture gradient theoretically. However, calculating stresses in different directions at a given depth is very complicated. Because of many uncertainties and assumptions involved on the rig-site calculations, the results should be taken as tentative.
Shut in Drill Pipe Pressure: We record SIDPP when the well is shut in upon getting a kick. We all know a kick takes place when formation pressure exceeds hydrostatic pressure. During the kick, certain amount of influx (formation fluid) enters into the hole with pressure. As soon as driller realizes the kick, he shuts the well by closing annulus using the annular BOP and records the SIDPP on stand pipe pressure console. Since stand pipe pressure sensor records SIDPP when the well is shut in; mudloggers can also read SIDPP in place of SPP. Recording SIDPP is crucial in order to kill a kick, as it is the amount of pressure that formation pressure has exceeded over hydrostatic pressure. So in order to kill a kick we have to increase hydrostatic pressure by the same amount as SIDPP. For example, we are drilling a well at 2500 ft tvd with 10 ppg MW and got a kick . Driller shuts the well and records 300 psi as SIDPP. This tells us that hydrostatic pressure needs to be increased by 300 psi in order to balance formation pressure. To do that we need to convert 300 psi into equivalent MW: 300 psi / 0.052 / 2500 ft tvd = 0.230 Eq MW. So to kill the kick we need to increase MW from existing 10 ppg to 10.23 ppg. Usually a safety margin of .1 to .2 ppg is added to provide sufficient balance. This will make final kill weight somewhere 10.33 to 10.43 ppg. SIDPP can also be used to calculate formation pressure accurately as below: Hydrostatic pressure + SIDPP = Formation Pressure. To calculate hydrostatic pressure = MW ppg x 0.052 x tvd Hydrostatic Pressure = 10 x 0.052 x 2500 = 1300 psi Formation Pressure = 1300 psi + 300 psi = 1600 psi
Shut in Casing Pipe Pressure: SICP is more or less same as SIDPP. While SIDPP exerts inside drill pipe and is recorded by standpipe pressure sensor, SICP exerts in the annulus of the hole and is recorded by casing pipe pressure sensor. This pressure is regarded as little inaccurate compare to SIDPP. The reason is that inside annulus we also have rock cuttings and gas apart from mud, While inside drill pipe we have uncontaminated mud (no cuttings and no gas), therefore it is SIDPP that is usually taken to calculate kill weight.
Differential Pressure: In mudlogging term, it is the difference between hydrostatic pressure and formation pressure in a wellbore. Differential pressure can create three different conditions in the hole: 1. Overbalanced Condition: When the hydrostatic pressure is greater than the formation pressure, the well is said to overbalanced. This condition helps prevent kicks but can lead to slow ROP or even lost circulation if hydrostatic pressure is too high 2. Underbalanced Condition: When the formation pressure is greater than hydrostatic pressure, the well is said to be underbalanced. This condition can increase the risk of getting kick but helps improve ROP. 3. Balanced Condition: When hydrostatic pressure is equal to formation pressure, the well is said to be in balanced condition. This (with slight overbalance) is an ideal condition to drill hole in an efficient and safe manner.
Conclusion: Mudloggers who have conceptual clarity of the different types of pressures encountered during drilling can perform their jobs more effectively, thereby contributing to safer and more efficient drilling operations.