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Effective solutions
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The main objectives and results of the first MPD trial job in the Yurubcheno-Tokhomskoye field are presented in this article. Based on the result of this job the conclusion for this technologies utilization in highly fractured carbonate formation can be assessed. The utilization of this technology is focused on increasing drilling efficiency, as well as the efficiency of exploration and production.
Introduction
The Yurubcheno-Tokhomskoye field is one of the biggest fields in Eastern Siberia. It is located in Evenkiysky district of the Krasnoyarsk Territory (see Figure 1).
The main exploration object in the Yurubcheno-Tokhomskoye field is the Yurubchenskaya deposit [1].
The deposit represents a highly fractured carbonate formation and is characterized by the presence of a gas cap (80% of the area) and separate near-vertical macro-fissures providing main oil inflow [2]. For the purposes of penetration into most of these fissures, the deposit has been directionally drilled providing wells with a horizontal section. The design HW length is 1000m.
Abnormally low formation pressure (ALFP) and highly fractured areas present in the formation do not allow for overbalanced HW drilling without circulating fluid (CF) loss.
According to calculations made using a specialized software, due to the mud rheology and specific well design (small diameter of the annular space between casing of 178 mm and drilling tool of 102 mm) the ECD varies in the range of 1.03 – 1.12 g/cm3, while the equivalent pressure of the productive formation does not exceed 0.94. As such, the required back-pressure is applied to the formation during circulation. However, the formation pressure is not more compensated in such static conditions resulting in an active fluid inflow (gas or oil). At the same time, loss of circulation causes mud level and hydrostatic pressure drop in the well also resulting in an active fluid inflow (kick).
Until now HW were drilled using LCM under the conditions of Oil Based Mud (OBM) loss. This method may worsen the poroperm properties of a productive formation depending on volume of lost mud and LCM. The use of LCM of different fractions and compositions has not allowed recovery of CF circulation for the further deepening the well and achieving the design HW length.
The MPD system trial job was implemented in August – September 2016 in order to solve the problem of creation of the required balance between CF (mud) density and hydrodynamic pressure along the HH length, and to prevent loss of circulation and gas, oil or water inflows.
Description of MPD Technology
Mud density for conventional drilling is chosen so that the static gradient would be higher than the pressure of the penetrated formation. The system is open, and the mud returns to the formation with atmospheric pressure. During circulation, pressure applied to the formation will drop due to frictional loss.
Compared to conventional drilling technologies, which use the mud density for pressure management, MPD technology combines wellhead pressure, friction and mud density in order to balance the pressure of the penetrated formation. In this particular case, the mud density was adjusted by obtaining a multi-phase CF using inert gas (nitrogen). Multi-phase circulating fluid may be used to reach a BHP required to keep a near balance condition, when the hydrostatic pressure of mud drops to the level slightly exceeding the formation pressure.
The key feature of the MPD system [3] is its absolute tightness within Rig pump to Gas separator area. This allows control of all processes in the bottom hole while drilling and mitigation of the risk of gas, oil or water inflow.
Main components of the MPD system and their purpose are shown in Figure 2 and in Table 1 [3].
2. Results of Trial MPD System Implementation
This BHP range [3] was developed considering that the equivalent pore pressure is 0.93 (see Fig. 3). Accordingly, the chosen horsepower of rig pumps was 13-14 hp and the chosen capacity of nitrogen units was up to 30 m3/min, which would allow creating the required formation back pressure and avoiding any problems related to loss of circulation / inflow.
Fig. 4 shows the actual operating pressure range and ECD [1] for the target well bottom of 3786 m. In order to reduce the mud loss intensity, its density was reduced up to 0.86 g/cm3, and the horsepower of rig pumps was limited to 11-12 hp. The formation gas inflow intensity was 10 m3/min.
One of the priority tasks of the MPD system is to reduce the mud (OBM) loss volume while drilling in fractured formations. Fig. 5 compares the specified values of mud loss volumes in the field (average) and in the first MPD well, where significant reduction can be observed.
Conclusions and Recommendations
• Horizontal section [4] of 897 m in length has been successfully drilled in the Riphean productive formation, and the main tasks have been resolved: mitigation of pollution of the productive formation by lost mud and LCM, increase in drilling rate;
• Initially, drilling had to be performed using MPD technology. It was estimated that equivalent pressure in this area is 0.93. Given this, the mud density of 0.93 g/cm3 (OBM) and capacity of nitrogen units of 15-20 m3/min have been chosen, which allow achieving the ECD of 0.94-0.95 g/cm3. However, intensive losses has been obtained under these conditions, whereupon the mud density was reduced up to 0.86 g/cm3, and the further drilling was performed with formation gas inflow of 3 -10 m3/min. This allowed reducing the intensity of losses up to 2-3 m3/hr. These conditions in the well were achieved with ECD within the range of 0.78 – 0.82 g/cm3, and capacities of rig pumps and nitrogen units were 11 hp and 20 m3/min accordingly. Back pressure during circulation was 2 kgf/cm2, and during buildup was 24 kgf/cm2;
• A hydraulic communication channel was used for transmission of SlimPulse telemetry signal [4];
• Gas flow meter played a critical role for determination and adjustment of ECD while drilling. It was used for real-time determination of formation gas inflow, and the obtained data were used for hydrodynamic modeling of well conditions and determination of the required ECD.
• In order to apply the required back pressure during round trips into the well hole, a high viscosity pill (HVP) [3] was used, and the drilling mud was replaced by heavy drilling mud with density of 1.06 g/cm3.
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