Monday, November 27, 2017

Matrix Acidizing chapter 3

A challenge that must be faced in either lithology is diversion. As acid is pumped, it flows preferentially along the most permeable path into the formation. The acid opens these paths up even more, and less permeable, damaged zones are almost guaranteed not to receive adequate treatment. Some technique to divert the treatment fluid toward more damaged formation or damaged perforations is therefore mandatory.

There is a variety of diversion techniques. Treatment fluid can be directed exclusively toward a low-permeability zone using drillpipe or coiled-tubing conveyed tools equipped with mechanical packers. Alternatively, flow can be blocked at individual perforations taking most of the treatment fluid by injecting ball sealers that seat on the perforations. In carbonates, bridging agents such as benzoic acid particles or salt can be used to create a filter cake inside wormholes, encouraging the acid to go elsewhere. In sandstones, microscopic agents such as oil-soluble resins can create a filter cake on the sand face. Chemical diverters such as viscous gels and foams created with nitrogen are used to block high-permeability pathways within the matrix.  


If the principle of matrix acidizing appears straightforward, the practice is a mine field of complex decisions.  Service companies offer a vast selection of acid systems and diverters, and few people would design the same job the same way. In addition, matrix acid jobs are low budget, typically between $5000 and $10,000 an operation, so the careful attention given to planning much more expensive acid fracturing treatments is often missing. Matrix acidizing is traditionally carried out using local rules of thumb. Worse, jobs are poorly evaluated. 

The question that should always be asked before any other is "Why is the well under producing?" And then : " Will production increase with matrix acidizing?" Production may be constricted for a reason other than damage around the borehole. The only way to find out is through pressure analysis from the deep formation through the wellhead, using production history, well tests and analysis of the well's flowing pressures, such as provided by NODAL analysis.

The crud maxim that matrix acidizing will benefit any well with positive skin has several exceptions. Too low a perforation density, multiphase flow, and turbulent gas flow are some factors that cause positive skin in wells that otherwise may be undamaged. 
NODAL analysis, which predicts a well steady-state production pressures, refines this checklist. For example, by comparing tubing-intake curves -essentially the expected pressure drop in the tubing as function of production rate-with the well's inflow-performance curve -expected flow into the well as a function of downhole well pressure -one can readily see if the well completion is restricting flow. Comparing a NODAL analysis with actual measured pressures also helps pinpoint the location of any damage. Damage does not occur only in the formation surrounding the borehole. It can occur just as easily inside tubing, in a gravel-pack or in gravel-pack perforation tunnel.


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