This Review presents a comprehensive literature review of an important class of kinetic hydrate inhibitors (KHIs) that is based on cyclic amides (lactams). The major aspects of the KHIs, such as their synthesis, inhibition mechanism, toxicity, biodegradability, performance, and cloud point, are thoroughly discussed. Data for 70 KHIs made of homo/co/terpolymers from 330 experiments are collected and evaluated for performance. The effects of the inhibitor concentration, molecular weight, monomer ratio of the co/terpolymers, and 35 different synergist chemicals on various KHIs are also studied. In conclusion, the top 10 KHIs with the highest performances that had a cloud point above 70 °C are presented. The copolymer (1:1) N-vinylpyrrolidone/N-vinyl-caprolactam (VP/VCap) is found to have the highest induction time (IT) of 13 to 14 h at 0.25 wt % and at a cooling rate of 1 °C/h. Some KHIs like Inhibex BIO-800 and poly­(N-vinyl azacyclooctanone) (PVACO) also have a very high ITs (17 and 13.5 h, respectively) at 0.25 wt % and at a cooling rate of 1 °C/h, but they have low cloud points (<25 °C). Guanidinium salts (n-Bu6GuanCl/n-Bu6GuanBr) and a phosphonium salt, (n-Pe)4PBr, are found to be the best synergists for the cyclic-amide-based KHIs. When used at 0.15 to 0.30 wt % along with the KHIs, they further increase the IT by 3–5 h.

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Three plant-based polysaccharides, pectin, k-carrageenan, and guar gum, are investigated as synergists with four kinetic hydrate inhibitors (KHIs), polyvinylpyrrolidone, polyvinylcaprolactam, Luvicap55w, and HIOP. The enhancement in the hydrate inhibition performance is characterized by measuring the increase in the delay/induction time (IT) taken for hydrate nucleation and the reduction in hydrate growth after nucleation. Standardized constant cooling rate hydrate formation tests are performed. Experimental results provided by 0.5 wt % aqueous solutions of reference KHIs are compared with the results of aqueous solutions made with combining 0.25 wt % reference KHI with 0.25 wt % polysaccharide synergist. K-carrageenan showed exceptional inhibition synergy with all KHIs with ITs enhanced around 20–35% with different reference KHIs and reduced hydrate growth rates up to 90%. Guar Gum did not increase the IT provided by reference KHIs. However, it decreased hydrate growth rates by 77–90% of all KHIs. Pectin showed exceptional hydrate inhibition synergy with HIOP (commercial KHI), boosting its IT by 45%.

Summary To minimize formation damage caused by drill-in and completion fluids, solids must be sized to satisfy two important criteria. First, they must be large enough to not invade the rock, and second, they must be small enough to form filter cake that effectively filters drill solids and polymers from entering the formation. These criteria, when used together with the model presented in this paper, quantitatively determine the particle size that should be used in drill-in fluids for a given formation permeability, overbalance pressure, and mud formulation. A model is presented that estimates the depth and degree of formation damage caused by solids of widely different sizes present in drilling or completion fluids. The depth of damage and permeability loss is calculated after the invasion of the mud and also after flowback. The effect of the particle size distribution in the fluid, particle concentration, overbalance pressure, and permeability of the formation are studied. It is demonstrated that particle invasion and flowback processes are largely dependent on the particle size in the mud and the permeability of the formation. The results of the model are shown to agree well with mud filtration experiments. To better estimate the particle size distribution in drill-in and completion fluids, different methods for measuring particle sizes were investigated. These results show that the measured particle sizes can vary over two orders of magnitude depending on the technique used and on sample preparation. Based on a comparative analysis of several samples, light-scattering techniques are recommended for measuring the particle size distribution. Recommendations for sample preparation are also provided. Introduction Fluids are used in the wellbore while drilling and completing a well. These fluids are maintained at a pressure higher than the formation pressure to prevent the reservoir fluid from flowing into the wellbore. Because of this "overbalance" pressure, the fluid invades the formation and can cause formation damage. The invading particles, which were initially suspended in the mud, tend to plug pores and therefore reduce the rock permeability. The mud filtrate can interact with the formation minerals to cause mobilization and subsequent redeposition of in-situ fines, and induce wettability changes leading to a reduction of permeability. It is, therefore, important to minimize filtrate invasion as well as solids invasion in the formation. The damage caused by particles in the mud and the volume of filtrate loss is the primary focus of this paper. Background The main factors that determine formation damage attributable to particle invasion are:Particle size distribution in the mud.Formation permeability/pore size distribution.Concentration of mud solids.Overbalance pressure.Mud circulation rate and rheology. Glenn and Slusser1 studied the effect of injected mud particle size distribution on formation damage. They conducted filtration experiments on porous samples with muds containing different particle sizes. The experiments indicated that a certain particle size distribution in the mud is required for a given pore size distribution for minimum permeability impairment. Zain et al.2,3 recently studied the effect of formation permeability on permeability impairment. They conducted a series of filtration experiments on five core samples with absolute permeability ranging from 3 to 2000 md. Two common drill-in fluids were used in the experiments: A sized calcium carbonate and a sized salt mud. The experimental results showed less damage in cores of low permeability, with damage increasing with increasing core permeability. This data clearly shows that formation damage is a strong function of particle size distribution in the mud and the permeability (pore size distribution) of a rock. Jiao and Sharma4 studied the effect of particle concentration on permeability impairment. They conducted a series of drilling mud invasion experiments on Berea sandstone cores to measure the extent and depth of formation damage by mud filtrates and mud particles. Seven kinds of muds with different salinities and different dynamic filtration rates were circulated across the face of the core. The researchers showed that lower solids concentration results in more damage (i.e., lower return permeability and higher depth of damage). Singh and Sharma5 and Thomas and Sharma6 showed that while drill-in fluids can be designed to be relatively nondamaging, the presence of drill solids in the mud can result in significant permeability impairment. There are various statistical guidelines used in the industry to choose the particle size of bridging materials that can form an efficient external filter cake and minimize formation damage. Abrams7 proposed two rules of thumb for selecting the size and concentration of bridging agents:A median particle size of the bridging additive equal to or slightly greater than 1/3 of the median pore size of the formation.The concentration of the bridging agents must be at least 5% by volume of the solids in the final mud mix. Hands et al.8 proposed that the D90 (90% of the particles are smaller than size x) of the particle size distribution of the bridging agents should be equal to the pore size of the rock. In sizing solids for use in drill-in and completion fluids, in the past attention has been focused on minimizing the invasion of solids into the formation. The rules of thumb discussed above ensure that sized solids do not invade the formation and cause permeability reduction. However, as shown by experiments conducted in our lab,9 the primary damaging mechanism in sized salt and sized CaCO3 fluids is not damage induced by the particles (which do not invade the rock) but rather by invasion of polymer particles and drill solids that can cause significant damage. Fig. 1 shows experimental results, which clearly indicate that when only calcium carbonate particles are used, no damage is observed in the core, whereas the addition of a biopolymer (xanthan) or the use of the biopolymer alone induces significant formation damage.

External filter cakes are used to minimize fluid loss and solids invasion to a formation from drilling and completion fluids. Subsequently, the cake must be removed in order to increase the flow area and minimize skins, especially for open hole and gravel packed completions in horizontal and deviated wells. Experimental data is presented to show that the pressure required to initiate flow into the wellbore after building up a filter cake is affected by rock permeability, mud properties such as particle size and cake yield strength, flow back velocity, and overbalance pressure. Mud cake lift-off tests, mud particle size, and rock pore throat size distribution measurements were performed to understand the factors that contribute to cake removal. The mechanisms that control filter cake removal are discussed. In this study, it is clearly shown that the flow initiation pressure during flow back is controlled by solids invasion, i.e. internal formation damage rather than by the external mud cake. Flow initiation pressures show a minimum with increasing rock permeability due to two competing effects. Larger pore sizes result in smaller flow initiation pressures, however, more solids invasion increases ?Pfi. Higher overbalance pressures also increase the internal formation damage and flow initiation pressure. A simple model to calculate the flow initiation pressure during flow back is proposed. The model correctly predicts the experimentally observed trends with rock permeability, mud particle size distribution, extent of solids invasion, and the yield strength of the mud cake. The model provides a systematic method for designing fluids with low flow initiation pressures. It is also shown in this study that low flow initiation pressures do not imply complete cake removal. Cake removal is primarily controlled by the permeability of the cake and its mechanical properties. Introduction Experimental results on the flow initiation pressure and the return permeability for two commonly used drill-in fluids were presented in an earlier paper1. It was unclear why the flow initiation pressure decreases and increases again after reaching a minimum as the core permeability increases. It was also observed that ?Pfi increases linearly with the fluid flow back rate. This work presents additional experiments and analysis to explain the factors responsible for flow initiation pressure and mud cake lift-off. Measurements of rock pore throat size and mud particle size distribution were conducted to assist in this explanation. Experiments Mud Cake Lift-Off Tests. The experimental apparatus and the procedures for fluid and core sample preparation and data analysis for the mud cake lift-off tests have been reported in an earlier paper1. Mud cake lift-off tests were performed under different conditions to understand the factors that control the flow initiation pressure and the mechanisms of filter cake removal. Mud Cake Lift-Off Tests. The experimental apparatus and the procedures for fluid and core sample preparation and data analysis for the mud cake lift-off tests have been reported in an earlier paper1. Mud cake lift-off tests were performed under different conditions to understand the factors that control the flow initiation pressure and the mechanisms of filter cake removal.