Yulong Yang

Exact Solutions for Nonlinear High Retention-Concentration Fines Migration

The paper discusses migration of natural reservoir fines lifted by high-rate or low-salinity water injection. The previous papers used linear analytical model, which is valid for low retention of mobilised fine particles in order to determine the model parameters from breakthrough fines concentration and pressure drop across the core during laboratory corefloods. The current work derives exact analytical solutions for the nonlinear case of high retention-concentration fines migration. The solution exhibits uniform profiles of suspended and retained concentrations ahead of the particle front and steady-state retained concentration behind the front. The obtained type curves allow distinguishing between linear and nonlinear fines migration. The laboratory data exhibit close agreement with the nonlinear model predictions, whereas the linear model poorly matches the laboratory data.

Fines Migration in Aquifers and Oilfields: Laboratory and Mathematical Modelling

Migration of natural reservoir fines is one of the main causes of formation damage in oil and gas fields. Yet, fines migration can be employed for enhancing reservoir sweep and water production control. Permeability decline due to fine particles’ detachment from reservoir rocks, mobilisation, migration and straining has been widely reported in the petroleum industry since the 1960s and is being researched worldwide. The topic of colloidal-suspension flows with particle detachment is also of wide interest in environmental, chemical and civil engineering. The current work begins with a detailed introduction on laboratory and mathematical modelling of fines migration, along with new mathematical models and experimental results. Each of the next three sections explores a particular cause of fines mobilisation, migration and straining. Section 2 covers high flow velocity that causes particle detachment accompanied by consequent permeability decline. Section 3 covers low-salinity water injection, where the decreased electrostatic attraction leads to particle mobilisation. Section 4 covers the effect of high temperature on production rate and low-salinity water injection in geothermal reservoirs. We attribute the long permeability stabilisation period during coreflooding with fines migration, to slow fines rolling and sliding and to diffusive delay in particle mobilisation. We derive the analytical models for both phenomena. Laboratory fines-migration coreflood tests are carried out, with the measurement of breakthrough fines concentration and pressure drop across the whole core and the core’s section. Treatment of the experimental data and analysis of the tuned coefficients show that the slow-particle model contains fewer coefficients and exhibits more typical strained concentration dependencies of the tuned parameters than does the delay-release model.

Formation Damage Challenges in Geothermal Reservoirs

Abstract Decline of well productivity due to migration of fine particles is a well-known phenomenon occurring during exploitation of geothermal reservoirs. We performed several laboratory fines migration tests using natural reservoir cores, taken from geothermal fields. The pressure drop along the overall core and the cumulative concentration of fines in effluent streams were measured during laboratory coreflood tests with piecewise constant decreasing ionic strength of the injected fluid. Core permeability stabilized only after 100–1000 pore volumes of the flowing fluid were injected. This suggests that mobilized rock particles move significantly slower than the carrier fluid. SEM-EDX (scanning electron microscope coupled with the thin film energy dispersive X-ray analysis) analysis of the produced fine particles shows that kaolinite and illite/chlorite are the main minerals causing permeability decline. Mathematical modeling showed that decreased viscosity of water and weakened electrostatic attraction force due to temperature rise competitively affected the particles attached to a grain surface. The micro-modeling of mechanical equilibrium of a fine particle attached to the surface of a sand grain shows the domination of the electrostatic attraction of fines to the surface of sand grains over the detaching drag force. Therefore, elevated temperature leads to an increased particle mobilization and consequent permeability damage. A newly developed “velocity–ionic strength” translation procedure determines the dependency of the maximum retention function (MRF) on the fluid velocity using experimental coreflood data corresponding to piecewise decreasing fluid ionic strength. Experiment-based evaluation of velocity and temperature dependencies on the MRF is demonstrated for specific conditions of geothermal reservoirs. Laboratory-based mathematical modeling on fines migration predicts the decline of geothermal well productivity during its exploitation. The derived mathematical model for one-dimensional flow with varying fluid ionic strength leads to a good adjustment of experimental breakthrough particle concentration and core permeability data, with tuning model coefficients falling within the common intervals. Size distribution of multisized particles is calculated using the obtained MRF for such particles. Temperature rise weakens the electrostatic attraction on particles attached to the rock surface much more than decreases the detaching drag force due to decline of the viscosity of water. As a result, one observes that an increased detachment of fine particles yields severer permeability damage, usually observed during exploitation of geothermal wells at high temperatures. Excellent agreement with a coefficient of determination R2=0.99 is observed between calculated well impedance results for a field case using the experiment-based predictive model and the historical well impedance data. This suggests that geothermal fields experience greater formation damage caused by migration of fine particles than conventional aquifers and oil reservoirs.

A New Phenomenon of Slow Fines Migration in Oil and Gas Fields (Laboratory and Mathematical Modelling)

Fines migration causes significant permeability damage, due to mobilisation of particles at increased velocities, their migration in pores followed by straining at pore throats and attachment to pore walls. Numerous coreflooding tests with piecewise increasing rates are conducted. There are two main features of these tests: the first is long-term injection, which allows calculating permeability stabilisation time; the second is pressure measurement at intermediate points, allowing for evaluating the permeability profile along the core. The impedance data obtained from experiments are matched with the results from analytical model. It shows that the mobilised particles move with velocity much smaller than the carrier fluid, yielding long time for permeability stabilisation. It contradicts the classical filtration theory, which indicates the fines are transported with the carrier fluid velocity.

Modelling of Slow Fines Migration and Formation Damage During Rate Alteration

Fines migration involving particle detachment in reservoirs often leads to severe permeability damage. It is the consequence of straining of the detached fines in relatively narrower pore throats. Many laboratory coreflood tests indicate that the time of permeability stabilisation can reach hundreds or thousands of pore volumes injected. However, the classical filtration theory assumes that the mobilised fines are transported by the bulk of the carrier fluid, thus the permeability stabilises after one pore volume injected. The current paper attributes the stabilisation delay to the slow drift of the released fines close to the rock surface. We propose the system of flow equations for fines migration in porous media taking into account the velocity of particles lower than that of the fluid. An analytical model for one-dimensional flow with particle mobilisation and straining during piecewise increasing flow rate is obtained. The laboratory data are in good agreement with the results of mathematical modelling. The effective particle speed is 500-1000 times lower than the water velocity.

New Laboratory Method to Assess Formation Damage in Geothermal Wells

The new method to assess permeability damage in geothermal reservoirs and predict well productivity decline is presented. The laboratory methodology developed aims to determine permeability decline from mobilisation, migration and straining of natural reservoir fines. Laboratory coreflood testing with constant and stepwise decreasing ionic strength has been performed with measurements of the pressure drop along the core and accumulated effluent particle concentration. Stabilisation of rock permeability occurs after injection of numerous pore volumes, suggesting slow drift of mobilised particles if compared with the carrier water velocity. Low ionic strength water increases electrostatic repulsion forces between clay particles and sand grain surfaces, further mobilising particles and resulting in formation damage. Kaolinite and illite/chlorite mixed layer clay minerals are identified by SEM-EDAX analysis and are the minerals primarily responsible for the permeability damage. The competitive effects of decreasing water viscosity and weakening electrostatic attraction on the attached particle concentration during temperature increase have been observed. The micro-modeling of the fine particle mechanical equilibrium shows that the water viscosity effect on the fine particle attachment dominates. It results in decreased fines detachment and permeability decline at high temperatures.

Prediction of Productivity Decline in Oil and Gas Wells Due to Fines Migration: Laboratory and Mathematical Modelling

Suspension-colloidal transport in porous media with the particle detachment usually exhibits a significant permeability decline. It occurs due to mobilisation and migration of detached colloidal or suspended fines with their straining in thin pores of the rock. Numerous laboratory coreflood tests show that the time for permeability stabilisation counts for hundreds of injected pore volumes, while the classical filtration theory assumes the released fines transport by the bulk of the carrier fluid yielding one pore volume injection to stabilise the permeability. In the current paper, the stabilisation delay effect is explained by slow drift of the mobilised fines near to pore walls. The basic flow equations for a single-phase particle transport in porous media with velocity lower than the carrier fluid velocity are proposed, and the analytical model for one dimensional flow with particle release and straining under the piece wise increasing velocity is derived. The laboratory data are in a good agreement with the results of mathematical modelling. The analytical model for well inflow performance is developed. It successfully matches several field cases.