What IS Reliability - Part 3

D). Application:?????????????? When it comes to evaluating the application for an artificial lift system, having all of the relevant details are crucial. Many times, an operator or end user gives some basic, generic, or whole-field data that will end up in a specific application. The results are such that the artificial lift method chosen may not be the best for those “generic” conditions and hence its performance is less then desired as well as the propensity of failures to be higher.

Well characteristics such as casing/tubing size and weight, sucker rod size and grade, well survey details, perforation depth, bottom-hole temperatures; fluid characteristics such as oil API Gravity, water cut, presence of H2S or CO2, solids production, gas gravity, presence of corrosion, scale or paraffin; production data such as static and flowing bottom-hole pressures, production rate, Productivity Index (PI), or Inflow Performance Relationship (IPR).

Improperly evaluating the application data can and will result not only in the high probability of installing the wrong artificial lift methods or size, but also increasing the chances that the artificial list method chosen may not operate effectively or efficiently thereby raising the chances that a failure will occur.

E). Well Profile/Completion:?????? To expand a bit on what was briefly mentioned above in the Application discussion, the well profile and completion make up are critical not only in evaluating the application, but also to determine what artificial lift method(s) may or may not be applicable as well as what well profiles and completions can have a tendency to create problems which eventually rise to the surface in the way of an artificial lift system failure.

Casing size and weight will determine what diameter of equipment can and should be installed. For equipment such as Electric Submersible Pumps (ESPs), there needs to be adequate room for the MLE as well as clearance between the motor OD and casing ID in order to have adequate flow past the motor for cooling. Casing dimensions combined with the well survey and presence/location of high DLSs, will affect not only what equipment will safely drift these areas, but also where the equipment can be landed. A maximum DLS of 2 °/100 feet in which an ESP should be landed is a typical “rule of thumb”, however many companies exceed this. The tubing size and weight as well as the tubing collars used needs to be evaluated against the casing ID and the power cable used to ensure adequate clearances. Many times, cross-collar cable protectors are used to protect the power cable from getting damaged.

For Progressing Cavity Pumps (PCPs), the clearance between the OD of the pump stator and the ID of the casing is also critical so that the pump is not installed in a high DLS area thereby putting the pump (rotor and stator) in a bending scenario. At this point, there is no “rule of thumb” for the maximum DLS in which the PC pump can be landed, so I would suggest to evaluate this on a well-by-well basis. The tubing size for a PCP, is critical on several fronts. First, the tubing ID needs to be evaluated against the size of the sucker rods used as well as the rod couplings (full size versus slim-hole couplings). The assurance that there is adequate clearance is critical in reducing or preventing rod/tubing failures. When the well survey and the location and severity of the DLS, the number and location of rod and/or rod coupling centralizers are important so as to reduce or eliminate rod/tubing wear and eventually a failure of either the sucker rods, the tubing, or both.

For Reciprocating Rod Pumps (RRPs), the tubing ID, sucker rod and rod coupling size, and DLS are also critical when looking to prevent or reduce failures. If the sucker rod size and rod coupling size are too large for the tubing used (ID and drift), then rod/tubing wear will occur. Rather than the rotational wear seen in a PC pump, it would be longitudinal. As with Progressing Cavity Pumps in high DLS areas, rod centralizers are to be used in order to reduce and/or eliminate excess rod/tubing wear.