Research on Overflow Monitoring Mechanism Based on Downhole Microflow Detection

The flow rate variation of the drilling fluid and micro-overflow loss is difficult to analyze. The purpose to prevent the occurrence of kick, lost circulation, and other complex conditions is not easy to be achieved.Therefore, themicroflow-induced annulusmultiphase flow rate and annulus pressure field model were studied, and a downhole microflow measurement system has been developed. A differential pressure type flow measurement was used in the system, and real-time downhole information was obtained to achieve deep, narrow windows and other safety-density complex formation security. This paper introduced a new bottom-hole flow meter which can measure the annular flux while drilling and monitor overflow and circulation loss. The accuracy and reliability of the MPD (managed pressure drilling) system can be improved obviously by applying the device; as a result, the safety of drilling is enhanced and the cost is reduced.


Introduction
Along with the rising of total demand of oil and gas resources, the explosion and development of world oil and gas is stepping into the peak gradually [1].Drilling engineering has become the focus of the oil and gas exploration and development.For the drilling operation is a hidden underground engineering, which has a large scale, high investments, and high risk, there is a lot of randomness, fuzziness, and uncertainty.Because of the misunderstandings of the objective situation or the errors in decision-making of the subjective consciousness, it may cause plenty of complex situations and serious disaster.The blowout accident is one of the most common and heavy loss safety accidents [2].Overflow is the precursor of blowout, which is caused by the unbalance formation pressure of the bottom hole during the drilling process.In the drilling process, overflow will invade the well due to various reasons, which will cause fluid contamination and drilling corrosion without timely detection.
Timely and early detection of overflow can prevent blowout accidents, reduce equipment damage caused by blowout, protect personnel and oil and gas resources, reduce environmental pollution, and ensure safe drilling, which plays an important role in the safe and effective development and the utilization of underground oil and gas resources [3].
Currently, to monitor the drilling operation, the mud level of the overflow tanks fixed on the ground is always detected.And then the formation fluid into the well of the overflow traffic can be determined from the mud level according to the principle of equal replacement [4].However, this approach has two disadvantages: the first one is that when the formation fluid early invades wells; the excess flow is too small and difficult to be detected, which leads to a serious lag; the other one is that the type of overflow cannot be determined.Therefore, the existing detection methods cannot detect overflow accurately and reasonable plan cannot be made [5].By monitoring the downhole microflow, the wellhead back pressure can be controlled and mud density can be finely adjusted to prevent from kick [6] and well leak disasters.Then, under safe and fast drilling in the condition of deep narrow mud window can be achieved.Therefore, it has become a trend in modern well-control technology to explore  and study new downhole overflow detection mechanisms and methods especially the technology of timely detection of overflow in the drilling process.This also ensures safe drilling and plays an important role in safe and effective utilization of oil and gas resources.
In order to measure the downhole annulus flow rate variation while drilling, a downhole annulus flow monitoring device must be designed.By measuring the annular flux while drilling and adjusting the open area of wellhead back pressure valve, the buttonhole pressure can be changed accordingly to control the overflow and lost circulation.This method can improve drilling safety and reliability and reduce drilling cost.The block diagram of microflow control system is shown in Figure 1.

The Model of Downhole Microflow Detection System
Based on the research on ultrasonic flow meter principle, electromagnetic flow meter principle, and differential pressure flow meter principle, differential pressure flow meter principle is used to realize the downhole microflow measuring [7].The choke element is shown in Figure 2. Considering passing ability of mud carried cutting, the limitation of length and cross-sectional dimension, and the requirement of pressure differential, the choke element is designed based on the structure of the stabilizer.
(1) Calculate the choke element equivalent diameter of the annulus :  1 is the annulus flow area of choke element, and 1 is the wetted perimeter annulus of choke element, and The average annulus velocity of the choke element is (2) Calculate the equivalent diameter between the drill pipe and the borehole annulus   : 2 is the cross-sectional area between drill pipe and wellbore annulus, and 2 is wetted perimeter of drill pipe and wellbore annulus, and The average velocity of the drill pipe annulus is (3) The annulus fluid Reynolds number of the choke element is For the structure stream  = 96/ Re.For the turbulent flow  = 0.015 ∼ 0.024.
The frictional head loss of every choke element is Then the local head loss produced by each choke element is 1 is the resistance coefficient sudden expansion of the local head loss,  2 is the suddenly reduced drag coefficient of local head loss, and  3 is flow resistance coefficient due to bending.
The differential pressure caused by each choke element is (4) The annulus fluid Reynolds number between the drill pipe and the borehole is The frictional head loss between the drill pipe and wellbore annulus per unit length is defined by For the structure stream  = 96/ Re.For the turbulent flow  = 0.015 ∼ 0.024.
(5) The pressure loss from the entrance point to the first stage chock is calculated by is the total length of 3-grade chock.The pressure loss before reaching the second stage chock is The pressure loss before reaching the third stage chock is The pressure loss before reaching the fourth stage chock is The pressure loss before reaching the th stage chock is The pressure loss after arriving the th outlet chock is The measuring device is mounted near the drill bit, and the drilling fluid flows through the throttling element on which a differential pressure can be generated [8].The differential pressure can be obtained by measuring the pressure of both ends of the chock element.The pressure is specified by annulus velocity of drilling fluid (or flow) as follows:

2𝑔
) . ( The structure diagram of the bottom-hole flow meter is shown in Figure 3.
In order to verify the correctness of the model [9], numerical simulations of the pressure distribution in the choke element are shown in Figure 4, which shows that the model fits both the calculation results and the experimental results [10].

Electrical System Design
The downhole microflow detection system is considered as an intelligent system with high reliability of the downhole pressure acquisition, storage, transmission, and processing.Its main parts consist of circuit of MCU and PC [11].
The circuit of MCU can accomplish signal detection, signal conditioning, data acquisition, and storage of downhole pressure.PC parts mainly complete data analysis and processing functions; the circuit of MCU and PC parts are connected through MWD tools [12,13].The two parts work together to accomplish the whole function of downhole microflow detection.

Results and Discussions
Applying bottom-hole flow meter in MPD can not only detect spills and leakage early but also take timely control measures.In order to verify the accuracy of the measurement instrument and the measurement result appropriate changes in the displacement of drilling fluid circulation are required, and the changes of the pressure values across throttling 22:00:00 00:30:00 03:00:00 05:30:00 08:00:00 10:30:00 13:00:00 15:30:00 18:00:00 20:30:00 23:00:00 01:30:00  element are needed to be recorded.The spot data of pressure value before and after chock is shown in Figure 5.
The curve indicates that the test data of the downhole pressure basically meets the requirements of site conditions, the general trend of the annulus pressure measured by the instrument increases with the increase in depth, and a series of processes from drilling such as going down, drilling, redressing, and short trip can be reasonably reflected in the curve.Compared with the logging data, it can be seen that the three distinct steps in the curve are caused by adjusting the slurry density; the middle part in the curve and the two apparent pressure drops before drilling are due to short trip; a lot of glitches in the curve represent the root process orders.Due to a large amount of data, some of the pressure fluctuations on the curve cannot be distinguished.Segmental analysis is needed for further analysis.
As shown in Figures  1.However, there are some fluctuations in the actual pressure curve, which indicates that the downhole test environment is disturbed in some degree during the test.

Conclusions
As  the overflow and drain wells to a minimum extent, and reduce disaster risks.

Figure 4 :
Figure 4: Numerical simulations of the pressure distribution in the choke element.
6, 7, 8, 9, 10, and 11, the variation trend of the actual pressure curve which is similar to that of

Table 1 :
The relation table between the mud displacement and the differential pressure.hardware to be solved.The downhole microflow measuring device developed in this paper measures downhole annulus flow rate while drilling and early detects kick and well leak.The application of the device will greatly improve the current process of drilling well control capabilities, decrease [14][15][16]ent of drilling technique, a new technique called MPD has been developed on the basis of conventional drilling and UBD (under balance drilling) in the past few years[14][15][16].This technique has been applied in some oil fields by now and good results have been obtained.However as a new technique, MPD still has some problems in theory and