(651c) Sand Management in Wellbore and Surface Facilities using HYSYS

Most of the oil and gas reserves are contained in weakly consolidated sandstone reservoirs. The sandstones around the boreholes undergo a gradual degradation due to the drilling operation, cyclic effects of shut-in and start-up, reservoir pressure depletion, and strength-weakening effect of water [1]. The high pressure gradient caused due to fluid flow leads to detachment of sand, and the sand particles get carried away to the wellbore with the fluid flow. The amount of sand produced can be few grams per cubic meter of reservoir fluid, causing only minor problems or so high that it results in erosion, or in some cases filling and blocking of the wellbore [1]. The sand produced from the wellbore can also damage the surface equipment such as pipelines and pumps.

In case of a high oil production rate, erosional damage can occur due to the increased impact velocity of sand on the pipelines and fittings. On the other hand, when the oil production rate is low, sand accumulation occurs in the wellbore as the fluid velocity is not high enough to carry the sand particles. In both scenarios, sand production is undesirable, and a complete sand control could be considered an absolute solution. However, a complete sand control could cost the oil and gas industry millions of dollars while also reducing the oil production rate. A better approach for dealing with sand production is to find an operability range for the oil production rate such that both sand accumulation and erosion are minimal. For this calculation, the sand transport characteristics have to be estimated at various hotspots at different operating conditions, which is time-intensive. Hence, a tool is required, such that provided with the operating conditions, it estimates the sand transport characteristics under different sand production rates for a given wellbore and surface facility.

This work focuses on building frameworks to estimate the sand transport probability in the wellbore, the extent of sand particle erosion in the wellbore and fittings, and the rate of sand accumulation in the surface facilities. The framework is implemented within HYSYS Spreadsheets for ease of use. A complete model of the wellbore and surface facilities is simulated in HYSYS. The fluid properties necessary for the calculations mentioned above are imported from the simulation to the HYSYS spreadsheets. The framework uses these properties to perform the calculations and display the results.

In slurry transport, the critical velocity is defined as the minimum velocity in which the solids form a bed at the bottom of the pipe from fully suspended flows [3]. In the wellbore, when the fluid velocity is lower than the critical velocity for sand transport, sand gets deposited in the pipeline. As the deposition increases, the cross-sectional area of flow decreases, which leads to an increase in local fluid velocity and increased erosional damage. Hence it is crucial to ensure that the fluid velocity is greater than the critical velocity for sand transport. In the framework, the sand transport probability (STP) in the wellbore is estimated as the overlapping coefficient [2] between the normally distributed critical velocity of sand transport and the normally distributed actual velocity of the fluid in the pipeline. The critical velocity of sand transport in horizontal pipelines is calculated using the Oroskar and Turian model (1980), while that in the vertical pipelines is calculated using the Stokes’ law. For inclined pipelines, the critical velocity is estimated by considering the critical velocities for the horizontal and vertical cases as the horizontal and vertical velocity components and interpolating between them for the angle of inclination of the pipe.

The framework consists of erosion response models from DNVGL-RP-0501 to estimate the extent of sand particle erosion in the pipelines and fittings. In these empirical models, the amount of material loss from a surface depends on the characteristic impact angles, impact velocity, sand properties, geometry, and target area [4].

In the surface equipment, sand is separated from the fluid by letting it settle in the equipment and then removing it. Sand is carried away by the fluid when the settling time of sand is more than the residence time of fluid in the equipment. The framework consists of a Residence Time Distribution (RTD) model to estimate the fraction of incoming fluid with a longer residence time than the settling time of sand to determine the fraction of sand that has settled. Here, the settling time of the sand is calculated using Stoke’s law. The rate of sand settling and the rate of increase of sand bed height are also calculated.

The developed frameworks in the HYSYS Spreadsheets calculate and display the STP, extent of erosion, and rate of sand accumulation using the fluid properties imported from HYSYS Simulation to the spreadsheets. HYSYS Case Studies have been used to generate and plot the sand transport probability and extent of erosion results for a range of sand production rates using the frameworks. These results are used to determine the operability range for oil production with minimum sand accumulation and erosion. The rate of sand settling calculated for the surface equipment aids in deciding the operating conditions that ensure a high separation of sand from the fluid. The rate of increase of sand bed height calculated helps to monitor the sand build-up inside the surface facilities. This tool is used to carry out sensitivity studies of different parameters like sand properties, the volumetric fraction of sand, and different operating conditions on STP in pipelines, extent of erosion in fittings, and rate of accumulation of sand in surface facilities. It allows testing the performance of different proposed wellbore geometries in terms of sand transport characteristics to plan a new wellbore.

The tool was used to estimate the STP and erosion extent in a model wellbore. The wellbore consists of a series of horizontal or inclined pipe segments from the bottomhole, a vertical pipe segment that takes the oil to the surface, an elbow joint to connect the pipe segments, a tee-joint to represent the tree, and a choke on the surface. The estimated STP was higher for inclined and vertical pipes compared to horizontal pipes. The extent of erosion was the highest in the elbow. For an oil and water mixture having a flow rate of 22200 kg/h and 50% water-cut at 74 °C and 5400 psia carrying 10 pptb sand, the STP in a horizontal pipe decreased from 0.99 to 0.98, and the rate of erosion in the elbow increased from 3.77 nm/year to 6.29 nm/year with increasing the sand diameter from 60 microns to 100 microns. On the surface, for a vertical separator of 1.2 m diameter having the maximum level of liquid in the separator as 50% and the retention time as 5 min, the rate of sand accumulation increased from 3.45 mm/day to 7.79 mm/day with increasing the sand diameter.

References

[1] Rahmati, H., Jafarpour, M., Azadbakht, S., Nouri, A., Vaziri, H., Chan, D., & Xiao, Y. (2013). Review of sand production prediction models. Journal of Petroleum Engineering, 2013.

[2] Henry F. Inman & Edwin L. Bradley Jr (1989) The overlapping coefficient as a measure of agreement between probability distributions and point estimation of the overlap of two normal densities, Communications in Statistics - Theory and Methods, 18:10, 3851-3874.

[3] Oroskar, A. R., & Turian, R. M. (1980). The critical velocity in pipeline flow of slurries. AIChE Journal, 26(4), 550-558.

[4] DNV, G. (2015). Managing sand production and erosion. Recommended Practice DNVGL-RP-O501. DNV GL Company, Oslo, Norway.