Plug remediation

Hydrate dissociation is of key importance in gas production from natural hydrate reservoirs and in pipeline plug remediation. Hydrate dissociation is an endothermic process in which heat must be supplied externally to break the hydrogen bonds between water molecules and the van der Waals interaction forces between the guest and water molecules of the hydrate lattice to decompose the hydrate to water and gas (e.g., the methane hydrate heat of dissociation is 500 J/gm-water). The different methods that can be used to dissociate a hydrate plug (in the pipeline) or hydrate core (in oceanic or permafrost deposits) are depressurization, thermal stimulation, thermodynamic inhibitor injection, or a combination of these methods. Thermal stimulation and depressurization have been well quantified using laboratory measurements and state-of-the-art models. Chapter 7 describes the application of hydrate dissociation to gas evolution from a hydrate reservoir, while Chapter 8 describes the industrial application of hydrate dissociation. Therefore in this section, discussion is limited to a brief review of the conceptual picture, correlations, and laboratory-scale phenomena of hydrate dissociation. 

The following sections present three examples of kinetic phenomena (1) kinetic inhibitors, (2) antiagglomerants (AAs), and (3) hydrate plug remediation. These kinetic phenomena were determined by field and laboratory observations. They also point to the need for a comprehensive kinetics theory, from which hydrate nucleation and growth can be predicted for industrial utility. 

In addition to primary cementing of the casing and liner, there are other important well cementing operations. These are squeeze cementing and plug cementing Such operations are often called secondary or remedial cementing [161]. 

Sentinel holes are used as a simple form of thickness testing. A small hole of about I - 6 mm diameter is drilled from the outer wall of the piece of equipment to within a distance from the inner wall (in contact with the corrodent) equal to the corrosion allowance on the equipment (Fig. 9.11). The technique has been used even in cases where the corrodent spontaneously ignites on contact with the atmosphere. The philosophy is that it is better to have a little fire than a big one which would follow a major leak from corrosion through the wall. When the sentinel hole begins to weep fluid a tapered plug is hammered into the hole and remedial maintenance planned. Siting the sentinel holes is somewhat speculative although erosion at the outside of a pipe bend is often monitored in this way. 

Causes of Well Plugging and Possible Remedial Actions... 

Control of site equipment is always important for safety and operational concerns. Many sites are remote or not staffed on a daily basis. Remediation equipment should be fitted with failsafe systems to shut down the system in the event of failure, fire, or unusual conditions (such as injection well plugging). Alarm systems should be included, which may be as simple as illuminating a warning hght, or as complex as a teleconnection to a remote computer station or telephone alert to the operator s residence. Many commercial companies offer remote monitoring equipment. 

Impure Feeds - Impure feeds cause Claus and Stretford plant problems (3,16). The most troublesome impurities are NH3, HCN, and hydrocarbons. These can cause catalyst fouling, plugging, chemical losses, and unstable operations. Remedies include catalytic conversion of impurities, use of special burners (16), and the use of different AGR processes to reduce the amounts of these compounds in acid gases. 

Forty-six case studies of hydrate plug formation and remediation are recorded in Hydrate Engineering (Sloan, 2000). In every case, hydrate plugs were remediated. In addition, a rule of thumb is that most of the offshore flowline shut-ins are less than the 10 h no touch time, which requires no antihydrate operation before restart (J.E. Chitwood, Personal Communication, August 1, 2003). However, hydrate prevention methods are very expensive, as shown in the above Canyon Express and Ormen Lange examples, or in the fact that deepwater insulation costs are typically U.S.Sl million per kilometer of flowline. 

In the future, economic risk evaluation will guide the hydrate-plugging prevention philosophy. It is important to note that phase equilibria thermodynamics provide the current paradigm of hydrate avoidance, but risk management is in the domain of time-dependent phenomena or physical hydrate kinetics. The experience base with hydrate plugs and their remediation impacts the economic need for large amounts of insulation and/or thermodynamic inhibitors. 

To remove the blockage, the line was depressured on both sides of the plug. Then methanol was circulated into the line to accelerate the hydrate dissociation rate. After complete removal of the hydrates, the dehydrator was cleaned, inspected and restarted properly. The entire remedial operation required 36 h to complete. The major cost was the lost production time. 

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