(11) DYNOMOMETER CONCERNS
Downhole dynamometer cards can be generated by predictive and diagnostic wave computer programs. Those generated by the diagnostic program are best for trouble shooting as they are calculated from a measured surface card. Several aspects of the bottom cards are presented in discussed in this paper some of which are well known and used and some which are less recognized.
For cards showing gas fill, the location of the TV closing is not the location in the downhole stroke where liquid fill begins. However it is close for low pressure wells. The shape of the load release can indicate if the well has low or high intake pressure. The pump load is indicated if the card is scaled correctly and generated by approximately correct damping or drag coefficients along the rod. A delayed opening of the SV on the upstroke can indicate that the pump is not tightly spaced. The load release distance is incorrectly identified as an area where compression is occurring but this does not indicate compression in the rod above the pump but is where gas, is present, is being compressed below the TV and above the SV. Example are shown of the above.
If looking at only the downhole card, excursions below where the load pick-up began indicate a compression load in the rod above the pump. However high damping coefficients will reduce the apparent fluid load shown on the bottom hole dynamometer and will also reduce the amount the bottom card may be showing apparent rod compression at the pump so the damping coefficients have an effect on apparent compression at the bottom hole card and these effects are illustrated. The shape of the bottom card can suggest if the damping coefficients are low or high. In general the predictive programs do not indicate compression at the pump unless pump resistance is guessed by the user but the bottom cards can indicate compression but the magnitude of indicated compression also depends on input for the diagnostic computer program.
Both the predictive and diagnostic programs can, under certain circumstance, indicate compression uphole from the pump due to dynamic effects. This can occur even with no pump resistance or rod compression at the pump. Rod failure characteristics can show if the rod broke after repeated compression (pump resistance) but this is after the fact.
These factors and more are discussed and illustrated to hopefully make problem recognition easier for the operator.