ON THE DYNAMIC EFFECTS DURING UNDERBALANCED DRILLING OPERATIONS AND THEIR PREVENTION Z. Wang, R. Rommetveit, RF-Rogaland Research; R. Maglione, Agip; A. Bijleveld, KSEPL, Shell; D. Gazaniol, Elf Aquitaine Production RF-- *\~IJois rbceiveo SEP 29 938 OSTt DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. ABSTRACTS In most underbalanced drilling (UBD) operations, the underbalanced condition must be generated artificially by injecting gas into the well. Due to the high compressibility nature of gaseous phase and interruptions to the system, the flowing system is a non-steady state one, especially when jointed pipes are used. This is experienced by the varying liquid and gas flow-out rates, and spikes in the bottomhole pressure. These dynamic pressures have been observed and documented in field UBD operations. This paper will first present an extensive examination of the dynamic effects during an underbalanced operation. The dynamic effects are often associated with drilling operations, like starting/stopping circulation, gas injection kick-in, changing fluids circulation rates, making connections, tripping, and deployment of BHA and downhole tools. Secondly, we discuss measures that are necessary to avoid the excessive peak loading of the surface facilities, the excessive wellhead pressures, and accidental overbalanced situations downhole. These are developed based on the field experience and simulation results from a dynamic underbalanced drilling simulator. We also demonstrate how a dynamic underbalanced drilling simulator can be used to improve the understanding of the physical process involved and be useful in designing operations. 1 - INTRODUCTION Underbalanced drilling has been used increasingly to address many field and operational problems during the last several years. The advantages of applying underbalanced drilling are to reduce formation damage, avoid lost circulation, minimise problems with differential sticking, increase the penetration rate and bit life. Formation damage can occur in various operational stages. There are several mechanisms that can cause formation damage or permeability impairment during drilling and completion operations [1,2]. Invasion of particulate matter into the formation is one of the most important mechanisms and it is often associated with the overbalanced pressure from wellbore towards formation in an overbalanced operation. Consequently, when underbalanced drilling is used to minimise formation damage due to solids and filtrate invasion, it is essential that the underbalanced condition is maintained throughout the drilling and completion operations at all times. Production while drilling may indicate that the well is underbalanced in some points/intervals. But it does not guarantee that the entire open hole section is underbalanced. There are many factors that affect the effective pressure drawdown between the reservoir pressure and the bottomhole pressure. Fig 1 summarises the possible factors that may result in an undesired condition. These factors will be discussed later in details in the paper. Assuming that other parameters are constant, the pressure fluctuation during a UBD operation is the single most important factor that will cause uncertainty of the actual pressure conditions downhole. In this paper, we will first illustrate the dynamic effects using some field recorded data. Then, we will present a comprehensive analyses of the system and the causes of the dynamic pressures. We will also show how we can use a dynamic simulator to simulate this effect, to improve the understanding of the physical process, and eventually to lead to the development of better procedures to minimise the effects. The results presented in this paper is obtained in a joint industry project at RF-Rogaland Research supported by Norske Shell, Norsk Agip, and Elf Petroleum Norge. 2 - UNDERBALANCED DRILLING OPERATIONS If the underbalanced condition must be generated artificially, gas injection via either drillstring or a type of parasitic string is employed. Figure 2 shows a schematic of a UBD system representation. Since in an underbalanced drilling operation, production occurs simultaneously, a UBD operation becomes a combined drilling and production operation. Hence, it requires more elaborate planning and engineering. Assuming a steady state flowing condition without significant drillstring movement, the average bottomhole pressure will be mainly determined by the following factors: • Wellbore geometry, • Types of drilling fluid and injection gas, • Drilling fluid pump rate and gas injection rate, • Surface control procedures, • Injection methods, • Rate of reservoir production. • Reservoir fluids type, especially gas oil ratio. Since a UBD operation involves a non-linear two phase flow system and the number of parameters needing to he considered -in the olanoincL^aaeUsJa^^ normally require the use of a dedicated simulator. There are a few steady-state underbalanced drilling software tools [5] [6] and a dynamic underbalanced drilling simulator available [7, 8], The UBD operation must be designed such that it is possible to achieve underbalanced condition throughout the operation within the operational restrictions. Operational restrictions may include maximum possible fluids injection rates, hole cleaning, borehole stability, and operational window for the mud motor. The operation has to be in a stable operating range where the well pressures are not too sensitive to changes in normal control parameters such as gas injection rate, fluid rate, wellhead pressure, and reservoir pressure drawdown. It is equally important to be able to predict the amount of injection gas required for the operation, and the volume and the rate of production. Operations like starting/stopping fluid/gas injection, drillstring movement, and pipe connections will interrupt normal flow conditions in the well and hence cause pressure fluctuation in the wellbore. Procedures have to be developed for such operations to avoid excessive wellhead pressures and accidental overbalance situations due to these operations. 3 - FIELD EXAMPLES OF DYNAMIC PRESSURE In this section, we illustrate the dynamic well pressure by using the bottomhole pressure recorded while drilling. The first example is from a UBD operation in Parana Basin, Brazil [4], Well 1-FR-1-SC was vertical with 13 3/8” casing set at 744 m. The well was drilled using a conventional rig with jointed pipe and foam from 754m to 884 m. The use of foam is to increase the penetration rate. The bottomhole pressure was measured while drilling. In Figure 3 the gas injection rate, mud rate, and measured bottom hole pressure in terms of the equivalent circulating density (ECD) are shown for the period of drilling from 860 to 884m. We observed that the bottom hole pressure did not reach a steady state within the time required to drill one pipe joint. From Fig. 3 we also observed that the highest bottom hole pressure during the period is nearly 50% higher than the lowest bottom hole pressure during the period. The pressure oscillations vs. time were caused by interruption in flow condition due to operational requirements. For the period shown, gas and liquid injections were stopped to make connections. Fig 4 plots bottomhole pressure against time for an oil well drilled underbalanced [3]. The bottomhole pressure during drilling has a variation of approximate 5 bar. However, during a drillstring connection, the bottomhole pressure first falls below the average due to the loss of the frictional pressure and then increases sharply above the average when the circulation starts again. This pressure spike repeats again when a drill pipe connection is required again. In this particular case, if one intends to maintain the underbalanced condition 100%, the minimum drawdown will be approximately 2100 kPa. 4 - FACTORS CAUSING DYNAMIC PRESSURES In this section, we analyse the possible factors affecting the bottomhole pressures (BMP). By doing so, we may identify the causes that result in the dynamic pressures in a UBD operation and subsequently draw up measures to avoid the undesired pressure fluctuation. The flowing BMP may be expressed as: Pwf=PH + Pf+ Pace + P«* In the above equation, Ph is the hydrostatic pressure component which is a function of gas and liquid densities and gas void fraction. Gas density is strongly dependent on pressure and temperature. The gas void fraction depends on the gas and liquid flow rates. P, is the frictional pressure loss component. Pacc is the acceleration pressure due to fluid acceleration. Pwh is the wellhead back pressure depending on the surface control on the choke, gas and liquid rates, and surface pipe network. Therefore, all four components will be dynamic and dependent on time and the state of the system. When a disturbance is given to the system, e.g., changing in either gas or liquid rate, all four components will change accordingly. Depending on the design and the state of the system, a disturbance may be damped out quickly , or on the other hand, it maylead to the instability of the system. In a UBD operation involving gas injection, the BMP will also be affected by the interaction of flowing system elements, i.e., gas injection line (parasitic string or drillstring), wellbore, and reservoir (Fig 2). Due to the nature of the non-linear two-phase flow system, interactions between the system elements, and various disturbances during an operation, the wellbore