COMPARATIVE ANALYSIS OF EQUIVALENT MUD WEIGHT FROM AN ADVANCED HYDRAULICS SIMULATOR AND PWD TOOLS Authors: Christian Ferreira – Blass Molina – Halliburton Abstract Hole cleaning is one of the main factors to the success of drilling of horizontal and highly deviated wells. This is a key factor to take into account primarily in angle construction sections and sections with angles between 40 and 90 degrees. Simulating drilling parameters during the planning stage allows the operator to anticipate drilling bottomhole pressures (BHP) that are measured by pressure while drilling (PWD) tools. Through careful analysis of these pressures, problems related to inadequate hole cleaning can be promptly identified, especially in sections with angles above 40 degrees. Improper hole cleaning will immediately show an abnormal increase in pressure values registered by PWDs. Simulating parameters such as rate of penetration (ROP), pump rates, rotating and sliding drilling times, connection times, average drilling cuttings size and hole angles can allow the accurate prediction of BHP while drilling. In this way the necessary corrective measures can be taken to help ensure the operation is free of problems associated with hole cleaning. This document validates BHP values predicted by the simulator compared with the actual values obtained with PWD tools at different depths and hole conditions. Once simulator values have been accepted, the same process is used to determine the expected pressure window registered by PWDs under optimum hole cleaning conditions. Any abnormal upward deviation in the pressure values measured by the PWD is potentially related to inadequate hole cleaning. Introduction Six horizontal wells have been drilled during the present campaign in Block 16 in Ecuador. Information from PWD tools and an advanced hydraulics simulator has been compared from the fifth well, where a PWD tool was run. A comparison was made between the actual pressure values presented as equivalent mud weight in the annulus registered by the PWD tool and the predicted equivalent mud weights obtained by the simulator considering the annular cuttings load, connection times, sliding and rotation times, and hole angles. This information is used to optimize the drilling design for the future wells, and to predict expected drilling near bit pressures to be measured by PWD tools, and to identify any deviation of the normal pressure trend that would indicate poor hole cleaning. Being the 8 1/2” hole section of the angle construction section, usually a 30% sliding and 70% rotating drilling operation is performed. It is clear that hole cleaning is a big challenge during this interval. Drillstring rotation has been identified as one of the main assisting factors in hole cleaning while drilling this hole section. As a complementary hole cleaning action, a sweep plan was simulated to improve drilling fluid cuttings carrying capacity and to prevent cuttings beds formation. The main objective of the simulation process was to determine the best combination of drilling parameters to maintain appropriate hole cleaning and to calculate the expected EMW for correlation with the EMW registered by PWD tools. The results given by the advanced hydraulics simulator show cuttings loading as one of the main factors to consider when evaluating hole cleaning. Experience shows that annular cuttings load values over 3% represent poor hole cleaning conditions. Cuttings loading has a direct effect on equivalent mud weights registered in the annular section. Drilling of this hole section is very critical since the formations involved are very dissimilar with intercalations of clays, shale, depleted sands, and limestone. Mud chemical properties and the mud density window had to be carefully defined to prevent hole instability problems. The field crew has to be alert to identify any instability signals that appear in the drilling parameters or in the appearance of the drill cuttings. This observation is critical to differentiate hole instability problems and hole cleaning problems which can have common indicators in the drilling parameters such as torque and drag. Data Comparison For this study, comprehensive hydraulics modeling was performed each 100 feet while drilling the section and the results of equivalent mud weights are shown considering the annulus loaded with drilled cuttings and without cuttings that represents hole clean conditions. The calculated results from the advanced hydraulics simulator and the actual EMW values registered by the PWD tool were correlated. The EMW values from both sources and the difference between them are shown in Table 1. As can be seen in Table 1, the EMW accuracy obtained with the hydraulics simulator and its correlation with actual values registered with the PWD tool are remarkable. It is important to highlight that the difference shown in the last column of the table is between the EMW values registered by the PWD and the EMW values calculated by the simulator including cuttings loading which is the real situation while drilling. The EMW without cuttings values are given as a reference and should be the minimum EMW to be registered by the PWD tool if the hole is free of drilling cuttings. Table 1. PWD and Hydraulics Simulator Equivalent Mud Weights Data Comparison APWD Data Hydraulics Simulator Data EMW W/O EMW With Depth EMW MW Difference Cuttings Cuttings 8683 10.55 10 10.61 10.63 0.08 8789 10.74 10 10.61 10.79 0.05 8886 10.7 10 10.61 10.8 0.1 8989 10.73 10 10.61 10.79 0.06 9086 10.81 10 10.62 10.83 0.02 9193 10.94 10 10.72 10.92 -0.02 9296 10.98 10 10.72 10.93 -0.05 9396 10.94 10 10.73 10.93 -0.01 9489 10.99 10 10.73 11.04 0.05 9586 11.06 10 10.73 11.07 0.01 9689 11.21 10 10.74 11.203 -0.007 9789 11.23 10 11.04 11.28 0.05 9889 11.31 10 11.05 11.29 -0.02 10030 11.48 10 11.12 11.35 -0.13 10060 11.68 MWD Failure 10.1 11.43 11.56 -0.12 10162 11.56 10.1 11.44 11.58 0.02 10260 11.59 10.1 11.45 11.59 0 10362 11.55 10.1 11.57 11.8 0.25 10390 12.02 MWD Failure 10.1 11.57 11.75 -0.27 10490 11.71 10.1 11.58 11.82 0.11 10590 11.74 10.1 11.58 11.82 0.08 Some of the parameters included in the simulator calculation process are shown below: Pump rates Hole angle and well geometry Sliding and rotating drilling times and ROPs. Drilling fluid properties Circulating and connection times Cuttings size and density Pipe rotation A graphical representation of the values obtained is presented in Fig. 1. It is important to highlight the similarity of the results for EMW with cuttings calculated in the hydraulics simulator with the values registered by the PWD tool. The trend of the lines is very similar with main differences registered at two MWD failure events. The red line represents the expected EMW values considering that drilling fluid is completely free of cuttings or the hole is totally clean. APWD and Hydraulics Simulator EMW values 8250 APWD Data 8500 EMW W/O Cuttings 8750 MW: 10.0 ppg Dev 33 EMW With Cuttings 9000 MW 9250 Napo formation 82 dev. Depth, Feet 9500 9750 Tena Formation MWD Failures 10000 10250 Basal Tena Formation 81 dev. 10500 MW: 10.1 ppg 10750 11000 10.2 10.8 11.6 11.8 10.4 10.6 11.2 11.4 12.2 9.8 10 11 12 Equivalent Mud Weight, ppg Fig. 1 - EMW comparison between simulator results and registered values from PWD. Drilling Optimization Plan Below are shown several recommendations being included in the drilling fluids plan for the wells to be drilled. The hydraulics simulator should be used to find the optimum combination of pump rates, ROP, pipe rotation, connection and circulation times, sliding and drilling times, and drilling fluid properties to help ensure effective hole cleaning. Validate planning results from the hydraulics simulator every 100 feet to re-calculate the equivalent mud weights to be expected with actual field parameters and to correlate these results with EMW values registered by PWD tools. Any deviation of the expected trends should be carefully considered and evaluated. Higher EMW trends registered by PWD could be an indicator of poor hole cleaning. Define carefully the drilling plan per well considering preventive actions to avoid hole cleaning issues. This plan will include drilling parameters optimization by reducing sliding times, increasing pump rates and maximizing rotation of the drillstring as much as possible. Continue obtaining actual near bit pressure values while drilling and continue correlating these values with results calculated by simulator to narrow the EMW expected window under optimum hole cleaning conditions and define corrective hole cleaning action plan if needed. Keep good communication and understanding of drilling parameters among all members of drilling team to take promptly action when parameters indicate potential hole cleaning issues. Use the hydraulics simulator to predict annular cuttings load. This cuttings load has a direct effect on equivalent mud densities. Cuttings load values over 3% usually represent poor hole cleaning conditions. Simulation Parameters and Results The figures below represent part of the drilling optimization process. These figures show the effect of changing drilling parameters on annular cuttings load and equivalent mud weights. The main objective in this simulation is to find the best combination of drilling parameters to optimize drilling times and hole cleaning. Besides the well geometry parameters and drilling fluid rheological properties, drilling parameters considered for this example are as follows: Mud weight 10 ppg. ROP: rotating @ 50 rpm: 60 feet/hr - - Sliding: 30 feet/hr Circulating time before connection 5 minutes rotating pipe @ 50 rpm Connection time 5 minutes. Average cuttings size: 0.2 inches. Using these parameters, the simulation process was performed considering several changes in drilling parameters to evaluate circulating time requirements, rpm, changes in mud properties, or corrective sweeps plan to help ensure good hole cleaning. Several figures show part of this drilling optimization study, where it is possible to evaluate the effect of drilling parameters like pipe rotation and sliding versus drilling times. It is important to highlight that a very comprehensive combination of drilling parameters was also evaluated but are not included due to space constraints. The following four figures are shown to describe the effect on annular cuttings load created by changing pipe rotation speeds and sliding/rotation drilling times. Drilling 75% Rotating and 25% sliding – Fig. 2 Drilling 100% Sliding - Fig. 3 Drilling 100% Rotating at 50 rpm – Fig. 4 Drilling 100% Rotating at 100 rpm – Fig. 5 Fig 2 – Annular cuttings load of 2.094% and the corresponding EMW of 11.339 ppg and the EMW value without cuttings load (hole clean) of 11.115 ppg. On the first column of the figure are shown the cuttings loads by hole section according to its geometry and hole angle. The second column shows average anular velocity for the fluid according to pump rate and hole section geometry. The third column shows the hole angle per section. The fourth column shows the calculated EMW trends with and without cuttings loads and the last column shows a graphical representation of the hole geometry. The upper right corner shows some of the parameters used for the drilling simulation and the calculated annular cuttings load percentage. Fig 3 – Situation considering an ROP of 30 ft/hr 100% sliding. On the top right corner appears in red an annular cuttings load of 5.394%. This value indicates that if the well is drilled with these drilling parameters, poor hole cleaning will be experienced. It is important to see also the effect of the high annular cuttings load on EMW: 11.616 ppg versus the EMW of 11.313 ppg shown in the previous figure considering 75% rotating and 25% sliding. Fig 4 – Situation considering an ROP of 60 ft/hr 100% rotating pipe at 50 rpm. On the top right corner appears an annular cuttings load of 1.798%. This value indicates that if the well is drilled with these drilling parameters, good hole cleaning will be experienced. It is important to see also the effect of the low annular cuttings load on EMW: 11.305 ppg versus the EMW of 11.115 ppg calculated if the annulus is free of cuttings. Fig 5 – Situation considering an ROP of 60 ft/hr 100% rotating pipe at 100 rpm. On the top right corner appears an annular cuttings load of 0.785%. This value indicates that if well is drilled with these drilling parameters, very good hole cleaning will be experienced. It is important to see also the effect of this very low annular cuttings load on EMW: 11.209 ppg versus the EMW of 11.115 ppg calculated if the annulus is free of cuttings. Conclusions Besides drilling fluid properties pump rates and hole geometry, other drilling parameters should be included in the drilling optimization process. These parameters include: ROP, pipe rotation speeds, rotation vs sliding times and cuttings size. Correlating EMW values with measured PWD values can allow prompt and correct decisions to help ensure good hole cleaning for drilling high angle sections. Drilling on paper using the advanced hydraulics simulator should be performed considering all possible scenarios to be faced during the actual drilling operation. All the time expended on drilling optimization planning will provide a better understanding of the actual well operation and will help facilitate accurate decisions. While drilling high angle hole sections, as sliding time increases, the tendency for cuttings beds formation increases and the annular cuttings load increases as well. In other words, as sliding time increases the efficiency of the hole cleaning has a significant reduction. The hydraulics simulator should be used to find the optimum combination of pump rates, rates of penetration, pipe rotation, connection and circulation times, sliding and drilling times, and drilling fluid properties to help ensure optimum hole cleaning. Drilling optimization results from the hydraulics simulator should be continuously re-calculated and correlated with actual EMW values registered by PWD tools. Any deviation of the expected trends should be carefully considered and evaluated. Higher EMW trends registered by PWD tools could be an indicator of poor hole cleaning. In reviewing the actual drilling operation data and correlating them with hydraulics simulator modeling results, the EMW base line for optimum hole cleaning should be achieved for each particular well situation.
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