Controlling formation damage caused by kaolinite clay minerals : Part II

During petrophysical and petrographical analyses of the core plugs, generally the laboratory technicians consider the formation damage mechanism to be the fines migration when they find kaolinite mineral to fill the pore space or line the same. However, during our research work on the fundamental causes of making fines and, consequently, the fines migration we actually identified a different mechanism to exist at low temperature which was not the fines migration. This new formation damage mechanism was the chemical oxidation of kaolinite, an amphoteric mineral similar in nature to Al(OH) 3 by sodium peroxide, Na 2 O 2 . We identified the chemical comminution through oxidation to be the process responsible for making formation fines from the mother kaolinite. The end products of the chemical reaction to a large extent were the small tubular structure of halloysite, the entangled shreds of the enlarged plates of dickite, and the separated, broken book stacks of mother kaolinite. In this work we have expanded the scope of the research to include the high temperature (250 degrees F) and long shut-in time (48 hours) effects on the chemical comminution of the kaolinite plates in a producing Tuscaloosa sand from central Louisiana. On the basis of our extensive analysis of data, we have concluded that under the high temperature and long shut-in condition: (I) the Chemical comminution of kaolinite has produced some amount of dickite, halloysite and some minor amount of illite in the Core # 3 with the subsequent formation damage. We measured the damage to this core of moderate permeability to be as much as 42.7 percent reduction in its original permeability, (2) the chemical comminution of kaolinite in Core # 4, a low permeability core, has only produced a small amount of dickite. Consequently, the reduction in the original permeability was low and only about 8 percent of the original core permeability. Therefore, presence of the kaolinite in the core does not necessarily mean an automatic formation damage problem by fines migration (3) again, similar to the low temperature and short shut-in time case, the main physical cause of the formation damage is the appearance of an in place large volumetric concentration of the entangled, shredded filaments of dickite plates (Core # 4) and the hollow tubes and/or curled plates of the halloysite within the pore space (Core # 3 ). This pore space was previously occupied by a small volume of the mother kaolinite book stacks, (4) although the kaolinite book stacks appeared to be separated in Core # 4, contrary to some petrographers beliefs, they did not cause appreciable fines migration type formation damage problem, (5) again, similar to the low temperature case, it appears that the main chemical cause of the formation damage is the appearance of a powerful oxidizer on the surface of kaolinite clay. In our case, this powerful oxidizer is sodium peroxide, and (6) to control the formation damage caused by kaolinite it is only logical to lower the pH to 8 or less, prevent aeration of the fluids in contact with clays like kaolinite and avoid the conditions which promote the formation or precipitation of oxidizers in the reservoirs.