THE EXCLUSION OF WATER FROM OIL-SANDS.
Water, by reason of its greater specific gravity, displaces oil and gas. Therefore it is of first importance that the water seep ing into the hole from water-bearing strata nearer the surface be prevented from flowing down the hole to the productive measures. Otherwise, when it has reached the latter it will displace the oil and gas, pushing them ahead of it, and eventually spread for a con siderable distance throughout the measure. The readiness with which it travels laterally varies with such factors as the density of the oil, porosity of the sand, etc., but even with very heavy oils the entrance of water into the sand soon makes itself known, not only in the production from the well where it has broken in but also in that derived from the nearby wells. Thus it is that the careless ness of one operator may lead to the ruin of an entire district, even though all the other operators have exercised every effort to pre vent the water from reaching the sand.
The importance of this subject is beginning to receive the at tention it warrants, but not until much damage has been done in the older fields, where the encroachment of water is without ques tion the most serious problem connected with the life of the wells. Many fields appear to have gone through the same stages. First the incipient appearance of water in the petroleum, then a grad ual increase in the percentage of the water content, until finally the field becomes irretrievably flooded or else so far gone that cor rective measures may be applied only at great expense. The dif ficulty connected with determining which well of a number in a zone is allowing the water to enter the oil measure, and the feeling of certainty expressed by each operator that it comes from another man's well, seem to point towards the imperative need for careful legislative action looking towards the effectual exclusion of the water at each well at the time it is being drilled and before the productive measures have been pierced, at a time when the thor oughness of the work may be satisfactorily tested. It seems too much to hope that supervision will be provided for and wisely administered, but if this is not brought about, the prospects for a vigorous and continued life of the newer fields are slight. The
presence of careless and incompetent operators, willing to take unwarranted chances in their efforts to hurry drilling operations, is inevitable in all fields, and in this situation the evil effects of their laxity are unfortunately not borne by themselves alone but also affect their neighbors.
All oil wells gradually decline in the amount of production, over periods of from a few months to several years. When water has made its appearance in a well the actual amount of moisture may be constant, but as the production of oil gradually falls off, the percentage of water will increase without there being an actual increase in the amount of water. Other wells act peculiarly in pumping all oil and then all water at intervals and such conditions are often hard to account for and equally difficult to remedy. However, when a well is pumping some oil it may safely be as sumed that all the water is being cared for, provided the origin of the water is from that particular well, but when the well pumps nothing but water the case is of course hopeless unless the dam age can be remedied. If the latter is found impossible, the entire hole should be plugged with cement to prevent the water spreading throughout the field.
The problem of water exclusion in its simplest form is merely that of inserting a string of water-tight casing, known as the `water string,' so that its bottom is tightly lodged below the lowest water-bearing stratum and above the top of the productive meas ures (Fig. 127) thereby sealing it off from descent below the casing shoe. In the districts where the distance between the two strata is not small, this may be easily accomplished in most cases. But in some districts the water stratum may be separated from the oil by only a few feet, and here the mechanical difficulties and need for care are great if the water is to be properly shut off and the full value of the productive measure realized.