As the search for oil moves into challenging areas such as deep water, where there is a narrow window between pore pressure and fracture pressure, the determination of the casing setting depth using kick tolerance needs to be more robust. The current industry practice for predicting the casing setting depth using kick tolerance assumes a constant geothermal gradient and ideal gas behavior in the calculations. The focus of this research is to study the effect of geothermal temperature variations and compressibility (Z) factor on the casing set depth design process.
The research method adopted to achieve this objective involves the development of an iterative Excel macro program for the prediction of casing set depth using kick tolerance which takes into account Z-factors and temperature gradient variations across subsurface formations take. The set depth for each case is predicted by comparing the fracture pressure equivalent density to the pressure generated within the wellbore during inflow circulation.
- Background
- Problem Statement
- Objectives
- Scope of Study
The ability of a well to withstand the pressure conditions generated during well control operations without fracturing the weakest formation.[1] The maximum shock volume that can be taken into the wellbore and circulated out without fracturing the formation at its weakest point (typically a foot of casing) given the difference between the pore pressure and the weight of the mud in use (shock intensity). An estimate of the amount of gas inflow at the bottom of the well that can be safely shut off and circulated from the well.
The conventional approach to selecting drill pipe set depth based on spade tolerance assumes a gas bubble flowing into the wellbore and uses it to calculate the pressure generated in the wellbore when the gas is circulated out of the well, assuming of a constant geothermal gradient and an ideal gas behaviour. Thus, the ideal gas law is used to calculate the volumes at different depths in the well.
The Kick Tolerance Concept
This will cause a significant increase in the maximum allowable casing shoe pressure during shut-in and therefore reduces the allowable kick tolerance in the wellbore. The value of kick tolerance calculated from equation (2) is compared with the value from equation (3), the smaller of the two values is used as kick tolerance. An algorithm, using an iterative procedure, defines the lowest depth for shoe setting using the kick tolerance concept.
The model showed that the higher the kick tolerance volume the greater the shallowest casing setting depth. The results show that the effect of temperature resulted in a higher kick tolerance volume compared to the current approach that assumes constant downhole temperature.
Gas kick Simulation Models
The early part of the petroleum industry was dominated by conventional wells and as such most well control procedures were developed for such wells, but the success of horizontal well technologies over the past few years has warranted research on many aspects of the subject. . The model can predict the behavior of the annulus pressure during the circulation of the wellbore. Determining the gas rise rate in the annulus for various well conditions is essential and fundamental to developing a more accurate kick tolerance calculation.
The rate at which free gas rises in the wellbore is a key parameter in the development of a gas kick in a well. This has an implication on the kick tolerance volume of the well, for the air/water system. Data was recorded using surface and subsurface sensors to study the development of the kick.
According to the results, the free gas velocity is a function of the homogeneous velocity, the flow distribution coefficient and the slip velocity. The gas inflow was treated as a distributed bubbling mixture of gas and mud, which occupies a greater length of the annulus. From the results, it is seen that a good model of the gas dynamics during the kick enables the estimation of the distribution of the inflow, which allows for a more accurate estimation of the maximum shoe pressure compared to the single bubble approximation.
As mentioned earlier, the successes of applying horizontal well technologies to reservoir development have seen much research and investigation conducted on the subject. Nunes, Bannwart and Ribeiro [15] reported a mathematical model to predict the annulus pressure behavior during wellbore gas circulation in a more challenging deepwater environment. In the mathematical formulation of the model, considerations related to the effects of well geometry, frictional pressure losses, flow expansion and two-phase flow patterns are implemented using various forms of equations.
Theory on Gas Behaviour
The model was then used to investigate the effect of different surface measurements on choke pressure. The line separating the liquid phase and the gas phase is defined as the vapor pressure curve, and the line separating the solid phase and the liquid phase is defined as the melting point curve. The liquid and gas phases are the most important part of the diagram for petroleum engineers.
Increased liquid concentration in the envelope is seen at increasing pressure and at decreasing temperature. As the inflow is circulated out, it undergoes phase change, liquid begins to fall out of the gas mixture at reduced pressure and temperature, this liquid concentration increases further as the gas reaches the surface. This shows that during kick circulation conditions exist in the wellbore where the gas concentration increases as the inflow approaches the surface.
Charles [19] also found that the temperature and volume of a given amount of gas are directly proportional. It was found that ideal gas behavior is valid only over a limited range of pressure and temperature conditions. The curves on the graph have similar shapes, but the actual Z values are component specific.
The law of the relevant countries states that all pure gases must have a similar Z-factor if the pressure and temperature are related to the critical pressure and temperature of the gas. In view of this, it was deemed necessary to develop equations that allow extrapolation to conditions outside the chart area. From the comparison results, the authors revealed that the fitted Hall Yarborough, Dranchuk and Startling equations presented in this paper are within the engineering accuracy ranges.
Algorithm
Calculate the pressure at the depth of the casing using the drill method and compare it to the fracture density on the shoe.
Mathematical Equations used in the Modeling
The inflow height is calculated by dividing the gas volume at the point of interest by the annulus capacity factor. Pressure (psi), temperature and Z-factor at the bottom of the well = Pressure (psi), temperature and Z-factors at depths. Gas compressibility factors (Z) are calculated numerically using the Dranchuk and Abou-Kassem correlation shown below along with the Newton-Raphson iterative method.
After obtaining these values, they are then substituted into equation (20) to calculate the annulus pressure generated within the wellbore during kick circulation. This pressure is then compared to the fracture pressure at the desired depth to determine the casing depth based on the following criteria. This process is repeated at various depths starting from wellbore TD upwards to determine the depth of casing placement as depicted in fig 18.
These mathematical equations will be programmed in the excel macro using the procedure described in the flow chart (Fig 8). The code developed will then be used to study the effect of temperature variations and Z-factors on case design using shock tolerance.
Model Input Data
The pore pressures and fracture pressures of the area are shown in the table below.
Study Cases
Methane gas 0.6 S.G, critical pressure 667.8 psi and critical temperature 3430R is to be the feed gas.
Temperature Profiles
Results and Discussions
- Case 1: Industry Approach
- Case 2: Effect of Gas Compressibility (Z) Factor
- Case 3: Effect of Variations in formations geothermal gradients
- Case 4: Effect of varying geothermal gradients and Z-Factor
Inflow volumes increase as the inflow moves up the wellbore where temperatures drop. The amount of inflow increases significantly at the very low temperatures found at shallow depths. This is probably due to the higher amount of inflow caused by the low temperatures at these depths.
Using a real gas equation to model the inflow gas behavior, the inflow volumes in the 8 ½ inch hole section increase with decreasing temperature along the borehole wall as in Case 1, but the inflow volumes are lower than those from Case 1 as shown in Fig. 16. For the 12 ¼ inch hole section in Case 2, the inflow volume curve is similar to that obtained for Case 1 (Fig. 14), but the absolute values are larger than those from Case 1 as shown in Fig. 20. In this case, the inflow volumes in the 8 ½ inch hole section are lower compared to those from case 1 as shown in fig. 21.
The increase in temperature in case 3 is believed to be responsible for the reduction in inflow volumes compared to case 1, which has lower temperatures for this hole section. Therefore, the increase in geothermal gradient leads to reduced inflow volumes during circulation compared to case 1 there, by giving a shallower predicted cure depth for the 9 5/8 inch case. For the 12 ¼ inch hole section, the inflow volume increases as it moves up the wellbore from TD.
The inflow volume along the 81/2-inch hole section in Figure 24, used to estimate the installation depth for the 9 5/8-inch casing shoe, has a similar curve to Case 3, but with lower inflow volumes compared to Case 1 and Case 3 .The inflow volume for the 12 ¼ inch hole section shown in Figure 26 for this case shows that the inflow in Case 4 is also lower than those of Cases 1 and 3. The inflow volumes, although lower, did not produce any change in the predicted setpoint housing depth for housing 13 3/8 inches.
Conclusions
Recommendations
Redmann Jrr, "SPE-Understanding kick tolerance and its significance in drilling planning and execution.pdf." 1991. Vefring, "SPE 22558-Comparison of Results from an Advanced Gas Kick Simulator with Surface and Borehole Data from Full-Scale Gas Kick Experiments in an Inclined Well," 1991. Abou-Kassem, "JCPT73-03-03- Calculations of Z Factors for Natural gases using equations of state," Calculation of Z-Factors for Natural Gases Using Equations of State, 2000.
