In this chapter, I would like to emphasize the basic problems of theconventional drilling technology, introduced by Anthony Lucas at Spindletop,which encouraged the development of the Managed Pressure Drilling technologyand to state the basic problems that can occur while performing the ManagedPressure Drilling. Conventional drilling is based on the establishing thepressure overbalance state to the bottom hole high enough to avoid any possibleinflux of formation fluid during drilling. Briefly, the conventional drillingfluid system consists of mud pumps, standpipe, drill pipes, drilling bitannulus, return flow pipe and the surface mud tanks. This circulating system isalso usually called the open to atmosphere system. Open to atmosphere meansthat all of the circulating pressure from the mud pumps is lost during thecirculation through the annulus due to the fraction, and the outlet pressurefrom the wellbore is equal to the atmospheric pressure. When the mud has notcirculated the pressure that acts on the bottom of the hole is the hydrostaticpressure of the drilling fluid, and when circulating the resulting bottom holepressure is the sum of the fluid pressure losses in the well and thehydrostatic pressure of the drilling fluid.
This conventional drilling practiceworked in the past, but for today’s subsurface conditions it is not soapplicable. Nowadays we are drillingmore complex wells due to their increasing depths. These drilling environmentsare called HPHT environments (High pressure- High temperature).
These wells areusually characterized by the narrow mud window making the conventional drillingtechnology unfeasible. This is the first advantage in favor of the ManagedPressure Drilling. The objective of the technology is the optimization drillingprocesses by decreasing the invisible lost time, mitigation of drilling hazardsand making the drilling project more economically feasible. Because thistechnology decreases the invisible lost time the highest potential for use ofthis technology is for the offshore drilling due to the enormously high dailyrates. The second reason for the offshore application for the Managed PressureDrilling is the lower formation integrity and lower pore pressure.
The reasonfor this situation is the hydrostatic pressure of the seawater that is lowerthan the weight of the adjacent rocks when we are drilling onshore. The consequences of the lower pore pressureand formation integrity are the depleted zones and narrow mu window, whichmakes conventional drilling method inefficient in these conditions. Themajority of the MPD systems use algorithms to detect the kick and apply acertain amount of the Surface Back Pressure to control the well. The mostimportant the MPD system reduces drilling costs and limits cost uncertainty by: 1. Avoiding conventional NPT problemssuch as lost circulation, shale instability, kicks, nuisance gas anddifferential sticking,2. Increasing the ROP, bit life anddecreasing the tripping frequency and3. Enabling access to potential assets/reservoirspreviously believed to be un-drillable.Drilling-related issues such as excessive mud costs, slow rate ofpenetration, wellbore ballooning/breathing, kick detection limitations,difficulty in avoiding gross overbalance conditions, differentially stuck pipe,twist offs and resulting well-control issues contribute to defining theoffshore industry’s need for MPD technology.
Main conventional Drillinglimitations that are going to be addressed in this work are:1. Problems due to the mud weight2. Well control3. Cost-related Problems 3.1. Weighted mud Invisible Lost Time While Drilling conventionally, onlypossible ways to react to the to unpredicted subsurface environments is eitherchange the mud density or to change the fluid density. Change of the fluiddensity is time-consuming.
The weightingof the mud process is the cause of the Non-Productive Time and drilling coststhrough:1. Reduction in drilling rate of penetration because of the increase of theoverbalanced mud solids content 2. Increased drilling costs (mud cost and rig cost)3. Increased casing seats and stringsThe rate of penetration is definedand affected by the mud weight of the drilling fluid that is used. and the mudweight of the drilling fluid is the function of the cuttings concentration.Higher the mud weight, the lower is the ROP due to the so-called ?chip holddown effect?. This refers to the capacity of the pressure differential betweenthe mud and pore pressures to inhibit the process of crack growth from the crushzone out to the free surface and the subsequent lifting off of the new chipfrom the bottom of the hole. Clearly, the opening of this crack and the liftingof the chip against the bottom-hole pressure will be hindered if fluid cannotflow into the growing crack to relieve the partial vacuum generated as thecrack opens.
This process will take time and will depend upon how easily fluidcan flow into the growing crack. The term used in the industry to define thiskind of drilling problems is called Invisible Lost Time. This can be determinedonly after a thorough date evaluation on the larger number of wells andcomparison between these data.
Also, the increase the mud weight we need to adda certain amount of solids to achieve wanted mud density. This increases thecost of the fluid itself. These solids, barite, for example, are inert whichmeans they don’t interact with the mud. With this in mind, we need to alsoaddress the possible and very significant equipment wear where the bit, mudmotors, pipe washouts and pump failures can occur.
In the case of the narrowpressure windows, the increase of the mud weight means that probably we aregoing to exceed the fracture pressure of the rocks. The result is that we needthe additional casing string to overcome this problem. Increased number ofcasing seats means the increase of the total cost of well and also increase ofthe NPT. Also, an excessive number of casing strings can result with a wellwhose diameter at Total Depth- TD may be too small to accommodate productiontubing large enough to produce economic volumes of hydrocarbons.3.
2. Well controlIn the conventional well controlactions, main counteractions that can be taken to act against the kick is toweight up our mud, dependent on the type of kick circulating method that wechoose for that task, and the pump rate of our mud pumps. As previouslydiscussed the main problem when drilling HTHP wells is the narrow mud windowbetween pore pressure and fracture pressure of rocks that are being drilled.
Because of that, there is a large possibility that we can encounter acontinuous change between high pressure and low pressured zones in subsurfacerock. Narrow mud windows can especially define our project economicallyfeasible or unfeasible when we are drilling long horizontal sections orExtended Reach Wells. Extended Reach wells are wells whose ratio betweenMeasured Depth and True Vertical Depth is 2/1 or even higher than that.
This means that when we are drilling longhorizontal sections our pore and fracture pressure gradients stays the same butthe length of the well is larger and the resulting annular friction pressurelosses are even higher. This can cause lost circulation situations andpotentially break formation that is being drilled. The continuous changebetween the high-pressure zones and low-pressure zones can also cause thekick/loss situation problems. While drilling when we encounter the low-pressurezone there is a large possibility that loss of drilling fluid can occur. If thelosses of the drilling fluid aren’t recognized at write time than the fluidlevel in the well falls down, well is not kept full at all times and kickoccurs. Also If that low-pressure zone is followed by the high-pressurehydrocarbon bearing zone there is a large possibility for a kick to occur.Another source of high risks for offshore drilling can be badly performedShallow Drilling Hazard Assessment. The Shallow Drilling Hazard Assessment isset of procedures conducted to identify all of the problems that can occurwhile drilling shallow formations.
The main reason is to identify the potentialfor encountering the shallow high pressured gas zones or the nuisance gaszones.3.1.
3. Cost related problemsAs the deeper and more complex wellsare being drilled, the costs per well drilled are also increasing. Whendrilling HPHT well conventionally the only way to encounter the high pressuresis to increase the mud weight. To increase the mud weight of our drilling fluidto a certain point we need to add a desired amount of solids, usually inertones such as barite. Increasing the mud weight, we are narrowing our drillingmud window even more. The addition of inert solids can cause two things: 1. It can cause the losses of fluid and2.
It can cause the erosion of thetollsWhen drilling conventionally we canovercome the lost fluid situations by injection of the LCM- Lost CirculationMaterials, injecting the cement plugs or by running a cementing a casing in thezones of fluid loss. The action of adding the LCM materials or preparing andinjecting the LCM cement plug is time-consuming and with high daily rates ofthe offshore platforms it is very expensive and increases the costs of thewhole project. When we are running and cementing an additional casing string inthe wellbore the factors that need to be considered for an increase in cost arenumerous. Time-consuming actions such as making connections of the casingstrings, running casing strings into the well, cementation and waiting forcement to reach the desired pressure strength needs to be considered also. Inthe end, additional casing strings that are run in the well also increase thetotal price of our drilling program. There is also one effect on the cost thatcan be recognized only when the drilling data from the numerous wells are beingcompared and it is called the Invisible Lost Time. Conventional drillingpractice demands certain overbalance for safe drilling program.
Overbalanceaffects our ROP, it decreases it, and if after the data analysis is confirmedthat the overbalance pressure was unnecessarily held to high then we are takinginto account the Invisible Lost time as a factor that defines total costs ofour well. 3.2.
Problems of the Manage Pressure Drilling MethodThe Managed Pressure Drilling is adrilling method that is able to solve the problems regarding the Underbalanceddrilling and Overbalanced drilling, the problems of the method are not yetentirely emphasized in the literature. When the Managed Pressure Drilling isused the main factor prior to the drilling of the well that has to be clarifiedis how the well is going to be drilled. Most of the problems that are addressedin the literature are connected to the failures of the equipment, the timeneeded to install all of the additional equipment and well-planning process forthe Managed Pressure Drilling.3.2.1. Well-planning considerations, limits and problemsAs for every drilling planningprocess, the assessed advantages and consequences regarding the drilling methodthat is being used has to be evaluated.
The major risks and parameters ofconsideration while using Managed Pressure Drilling are:1. Pressure tolerances and mud windowof the formation2. Flow potential of the well Pressure tolerancesand mud window limit the mud weight of the drilling fluid that is going to beused for performing this action and also determines the maximum allowable surfacebackpressure that can be applied during drilling. Flow potential, in this case, has to beviewed from two perspectives, as a flow potential of the subsurface rock thatis being drilled and flow potential of the well that defines the ability forcirculating the kick out of the well. Flow potential of the rock that is beingdrilled is imp