Effect Of Pressure And Temperature Distribution To The Drilling Fluid .

CHAPTER 1INTRODUCTION1.1 Background StudyDrilling fluid performance is a major factor that contributes to the drillingoperation‟s success. The properties of the drilling fluid such as equivalent circulatingdensity (ECD), equivalent static density (ESD) and rheological properties alwaysassumed to be constant during the operation. This assumption can prove to be incorrectin high pressure/temperature wells with pressure and temperature variations. Drillingoperations in the formation with narrow gap between pore and fracture pressure marginsare very impossible to be done using conventional drilling method, with the slightchange in bottom-hole pressure conditions can lead to an increase in the NonProductive Time (NPT) caused by kick or fluid loss with possible blowout occurrence.For these reasons, a new technique has been introduced under Managed PressureDrilling (MPD) called Constant Bottom Hole Pressure (CBHP). Managed PressureDrilling (MPD) is an adaptive drilling process used to precisely control the annularpressure profile throughout wellbore (Vieira P., 2009). This technique enables thedrilling operation continued with the bottom-hole pressure is maintained constantwhether the fluid column is static or circulating. This concept referred to „walking thelines‟ between pore pressure and fracture pressure gradients. The loss of annulus flowingpressure when not circulating is counteracted by applied surface backpressure.According to J. Shubert (2009), the basic concept of Constant Bottom Hole Pressure(CBHP) is to accurately determine the change in bottom-hole pressure caused bydynamic effects and compensate with an equal change in annular wellhead pressure. Forthis application, it requires better wellbore pressure management and correct planning onthe drilling fluid design to be implemented.As the total vertical depth increases, there is an increase in the bottom-holetemperature and hydrostatic head of fluid column. These parameters have opposingeffect on ECD (Mc. Mordie et al., 1982). An increase in hydrostatic pressure cause1

increase in ECD due to compression but an increase in the temperature causes areduction in ECD due to the thermal expansion. Usually these effects are assumed to becanceling each other out in conventional drilling but for MPD applications, it was verysignificant and precise estimation of static and dynamic equivalent density is of essentialimportance for the drilling operation through narrow operation window wells1.2 Problem StatementDuring the drilling process, the drilling fluid temperature is not constant due tothe thermal phenomena present during circulation of the drilling fluid. There is heattransfer from the formation to the drilled hole due to the difference between geothermaland drilling fluid temperatures. According to E. Karstad et al (1998), the drilling fluiddensity is strongly affected by the formation temperature and annular pressure. The typeof drilling fluid plays an important role in drilling fluids behavior with changes intemperature and pressure profile. Considering the thermal expansion and pressurecompression effect, the rheological properties especially density of oil/synthetic baseddrilling fluids, changes significantly in high pressure/high temperature (HPHT) wells(Courtesy of Mullen et al., 2001).Hydrostatic pressure calculation in deep wells, with high bottom-hole pressureand temperature, requires a correction for the fluid density of each interval of the hole.Increasing temperature decreases the density of fluid, while increase pressure increasesfluid density. This phenomenon may be significant in Managed Pressure Drilling as thistechnique used pressure control system as the main indicators to control the bottom-holepressure keep constant while drilling operation especially for narrow operating windowwells.With the significant changes in drilling fluid density in term of equivalentcirculating density and circulating pressure, the fluid rheology and flow rate should beconsidered in order to predict the pressure loss throughout circulation system (Bazer D.,1991).Equivalent circulating density is become very important to be monitored2

especially in HPHT wells due to the effect of pressure and temperature profile in orderto avoid kicks and losses.The changes in equivalent circulating density due to the effect of pressure andtemperature during drilling operation brings some of the significant problems in annularpressure profile result in loss circulation of drilling fluids to the formation and invasionof the formation fluid into the wellbore. Without the proper consideration of theseeffects, it may result in an accurate estimation of bottom-hole pressures and incorrectapplication of MPD techniques as the reference values are not precise, in consequencesdealing with increase in Non- Productive Time (NPT). The precise determination of alleffects on density reduction by the formation pressure and temperature lead to minimizethe uncertainty when controlling drilling problems.1.3 Objectives1.3.1 To determine the rheological properties such as plastic viscosity (μ P) and yieldpoint (τY) of n-paraffin oil based mud as drilling fluid used with mud weight 14.6ppg by using rheological modeling.1.3.2 To simulate the equivalent circulating density (ECD) and circulating pressurewith pressure and temperature variations as the main parameters for ManagedPressure Drilling (MPD) applications using Landmark Wellplan.1.4 Scope of StudyThe study which is carried out in this project is to determine the effect ofpressure and temperature distribution to the rheological properties in term of density forn-paraffin oil based mud. The rheological properties of drilling fluids such as plasticviscosity and yield point are modeled using empirical model in term of pressure andtemperature. A Bingham Plastic model was used as the fluid rheological model for thisdrilling fluid in order to determine the rheological properties. This empirical model isused for precise correlation with the data using HPHT viscometer and Mercury FreePVT system experiment to show the relationship of rheological parameters for n-paraffinoil based mud. This model included the determination of rheological properties (plastic3

viscosity and yield point) and the fluid density changes with pressure and temperature asthe parameters. The data for rheological modeling was then used in the simulation of theECD and circulating pressure.The equivalent circulating density (ECD) and circulating pressure are determinedusing simulation by Landmark Wellplan with the different pressure and temperaturedistributions. These two parameters are very important as it indicates the hydrostaticpressure applied by drilling fluids to the formation at certain depth during drillingoperation. Other parameters also used in the simulation in order to generate ECD,circulating pressure and temperature profile for specific scenario such as circulatingtimes, pump rates and backpressures.The scope of study is mainly to determine the equivalent circulating density andcirculating pressure with down-hole variations in pressure and temperature for ManagedPressure Drilling applications and the use of rheological modeling to predict the fluidrheology behavior. In this project, author proposed to use n-paraffin oil based mud asdrilling fluids to determine the ECD and circulating pressure under variations ofpressure and temperature.4

CHAPTER 2LITERATURE REVIEWThe objective of this section is to review the literature in several areas related tothe objectives of the study. In order to accomplish the objective, previous researches andstudies were cited to gain knowledge and basic ideas about the project. Numerouspublications and researches have dealt with the behavior of density of drilling fluids inresponse to variations in pressure-temperature conditions. Various models have beenproposed in order to characterize this relationship, with some models being empirical innature, and others compositional. The compositional model characterizes the volumetricbehavior of drilling fluids based on the behavior of the individual constituents of thedrilling fluid.In the compositional model, the density of any solids content in the drilling fluidis taken to be independent of temperature and pressure. It is assumed that any change indensity is due to density changes in the liquid phases. It is also assumed that there are nophysical and chemical interactions between the solid and liquid phases in the drillingfluid. Hoberock et al proposed the following compositional model for equivalent staticdensity of drilling fluids. . (2.1)Application of the compositional model requires some knowledge of how thedensities of each liquid phase in the mud, usually water and some type of hydrocarbon,change with changes in temperature and pressure. The static mud density at elevatedpressure and temperature can be predicted from knowledge of mud composition, densityof constituents at ambient or standard temperature and pressure, and density of liquidconstituents at elevated temperature and pressure.5

Peters et al applied the Hoberock et al compositional model successfully tomodel volumetric behavior of diesel-based and mineral oil-based drilling fluids. In theirstudy, they measured the density of the individual liquid components of each drillingofluid at temperatures varying from 78-350 F and pressures varying from 0-15,000 psi.Using this data in conjunction with Hoberock et al‟s compositional model, they wereable to predict the density of the drilling fluids at the elevated temperature-pressureconditions.Isambourg et al proposed a nine-parameter polynomial model to describe thevolumetric behavior of the liquid phases in drilling fluids, which is applicable in theorange of 14.5-20,000 psi and 60-400 F. This model characterizes the volumetricbehavior of the liquid phases in the drilling fluid with respect to temperature andpressure, and is applied in a similar compositional model to that proposed by Hoberocket al. The model also assumes that all volumetric changes in the drilling fluid is due tothe liquid phase, and application of the model requires a very accurate measurement ofthe reference mud density at surface conditions.Babu compared the accuracy of the two compositional models proposed bySorelle et al and Kutasov respectively, and the empirical model proposed by Kutasov inpredicting the mud weights for 12 different mud systems. The test samples consisted of3 water-based muds (WBM), 5 oil-based mud (OBM) formulated using diesel oil andmineral oil. Babu found that the empirical model yielded more accurate estimates for thepressure-density-temperature behavior of a majority of the mud over the range ofmeasured data more accurately than the compositional model. He also concluded that theempirical model has more practical application because unlike compositional models, itis not hindered by the need to know the contents of the drilling fluid in question.Drilling fluids contain complex mixtures of additives, which can vary widelywith the location of the well, and sometimes with different stages in the same well. Thiswas especially apparent in the behavior of the drilling fluids prepared with diesel oil No.2. Different oils available under the category of diesel oil No. 2 that were used in the6

preparation of OBM‟s can exhibit different compressibility and thermal expansioncharacteristics, which were reflected in the pressure-density-temperature dependentbehavior of the fluids prepared with them. The drilling fluids also consist of drillcuttings from the formation rock cuttings during drilling operations. The temperatureand pressure of the annulus and wellbore might affected the thermal expansion andcompressibility of the drill cuttings of the formation. This will lead to changes incirculating mud density and circulation pressure of drilling fluids in wellbore andannulus.Research has also been reported on characterizing drilling fluid rheology at hightemperature/pressure conditions. Rommetveit et al approached their analysis of shearstress/shear rate data at high temperature and pressure by multiplying shear stress by afactor that depends on pressure, temperature and shear rate. Coefficients of thismultiplying factor are fitted to shear stress/shear rate data directly without extractingrheological parameters such as yield stress first. This eliminates the need to characterizethe behavior of each rheological parameter relative to pressure and temperature changes.They obtain an empirical model in which the effects of variation in all rheologicalparameters that describe fluid flow behavior are lumped together.Another approach to the analysis of temperature and pressure effects on drillingfluid rheology is to consider the effect of temperature and pressure changes on eachrheological parameter that describes the behavior of the fluid. The two most commonmodels considered for such an analysis are the Herschel-Bulkey/Power law model andthe Casson model which is an acceptable description of oil based mud rheology. Ofthese two models, the Herschel-Bulkley model is the most robust, as it is a threeparameter model as opposed to the Casson model which is a two parameter model. Inthe analysis performed by Alderman et al on shear stress/shear rate data, the HerschelBulkley/Power and Casson models were considered. The behavior of each rheologicalparameter in these models with respect to changes in temperature and pressure wasinvestigated. They studied a range of fluids covering un-weighted and weightedbentonite water-based drilling fluids with and without deflocculant additives.7

In order to estimate equivalent circulating density, it is important to take intoaccount the effects of temperature and pressure on fluid rheology. Two methods areproposed to accomplish this by Rommetveit et al. They propose a stationary or staticmethod and a dynamic method. In both methods, the contributions of hydrostatic andfrictional pressure losses in high pressure/high temperature wells to the equivalentcirculating density were considered. The variation in temperature vertically along thewell bore is taken into account for both models, and drilling fluid properties are allowedto vary relative to temperature.The dynamic method however, also takes into account transient changes intemperature as change in temperature over time. This effect is especially important inthe case where circulation has been stopped for a significant amount of time. Thedrilling fluid temperature will begin to approach the temperature of the formation. Oncecirculation commences again as shown in Fig. 1, the lower part of the annulus will becooled by cold fluid from the drill string and the upper part of the annulus will bewarmed by hotter fluid coming from the bottom-hole. During this transient period, fluiddensity and rheological characteristics can change rapidly due to rapid changes intemperature. Research on this effect is still at a very early stage and will not be takeninto account during this study.Figure 1: Schematic Diagram of Fluid in the Well bore at the Start of Circulation Rommetveit et al, 1997.8

Alderman et al performed rheological experiments on water based drilling fluidsoover a range of temperatures up to 260 F and pressures up to 14,500 psi, using bothweighted and unweighted drilling fluids. Rheograms were obtained for the water baseddrilling fluids, holding temperature constant and varying pressure, and vice versa. It wasfound that the Herschel-Bulkley model yielded the best fit to the experimental data.Other models that were investigated are the Bingham plastic model, and the Cassonmodel which some authors argue is the best model for characterizing oil-based drillingfluid rheology.For the Herschel-Bulkley model, it was found that the fluid viscosity at highshear rates increased with pressure to an extent, which increases with the fluid density,and decreases with temperature in a similar manner to pure water. Alderman et al foundthe yield stress to vary little with pressure-temperature conditions. The yield stressremained essentially constant with respect to temperature until a characteristic thresholdtemperature is attained. This threshold temperature was found to depend on mudcomposition. Once this threshold is reached, the yield stress increases exponentially with1/T. Alderman et al also found that the power law exponent increased with temperature,and decreased with pressure. This makes them to conclude that the Casson model willbecome increasingly inaccurate at these two extremes, which is at high temperature andlow pressure.The estimation of ECD under high temperature conditions requires knowledge ofthe temperatures to which the drilling fluid will be subjected to down-hole. As the fluidis circulated in the wellbore, heat from the formation flows into the wellbore causing thewellbore fluid temperature to rise. This process is more pronounced in deep, hot wellswhere the temperature difference between the formation and the well-bore fluid isgreater. The process is very dynamic at early times that are, at the commencement ofcirculation, with great changes in fluid temperature occurring over small intervals oftime.9

There are two major methods for estimating the down-hole temperature ofdrilling fluid. The first is the analytical method. This method assumes constant fluidproperties. Ramey solved the equations governing heat transfer in a well bore for thecase of hot-fluid injection for enhanced oil recovery. His solution permits the estimationof the fluid, tubing and casing temperature as a function of depth. He assumed that heattransfer in the well bore is steady state, while heat transfer in the formation is unsteadyradial conduction.Holmes and Swift solved the heat transfer equations analytically for the case offlow in the drill pipe and annulus. They assumed the heat transfer in the wellbore to besteady state. However, they used a steady-state approximation to the transient heattransfer in the formation. They justified this assumption by asserting that the heattransfer from the formation is negligible in comparison to the heat transfer between thedrill pipe and annular sections due to the low thermal conductivity of the formation. Theresult obtained by Holmes and Swift have been used in different situations and predictedsuccessfully the bottom-hole temperature using temperature logs. However, all thedeductions and analytical expressions used to determine the drilling fluid temperatureprofile in the annular space have been used basically in vertical well. That is verycommon to find more applications of directional wells in present using samemethodology. Acuna and Arnone obtained a mathematical expressions adjusted to anywell trajectory.The second method of estimating fluid temperature during circulation involvesallowing the fluid properties such as heat capacity, viscosity, and density to vary withthe tem

Drilling (MPD) called Constant Bottom Hole Pressure (CBHP). Managed Pressure Drilling (MPD) is an adaptive drilling process used to precisely control the annular pressure profile throughout wellbore (Vieira P., 2009). This technique enables the drilling operation continued with the bottom-hole pressure is maintained constant whether the fluid .