Rock classification

1. Before reading the text, study the vocabulary:

to evolve	выделять, выявлять
to penetrate	проникать, проходить
tool jamming	заедание, защемление инструмента
brittleness	ломкость, хрупкость
crude methods	неточный, неразработанный метод
to suffice	быть достаточным
rough calculations	приближенное, грубое вычисление
the drilling-rate index (DRI)	коэффициент скорости бурения
the bit-wear index (BWI)	индекс износа долота
inversely proportional	обратно пропорциональный
vice versa	наоборот
to exert	оказывать давление, влиять
offset wells	соседние скважины

2. Write down the transcription of the following words and practise their pronunciation:

• primarily

• straightness

• to suffice

• measurement

• inversely proportional

• vice versa

3. Read and translate the text.

Many attempts have been made to classify rocks on the basis of their drillability, and various measuring methods have been evolved with the aim of ma­king it possible to predict productivity and tool wear by carrying out a laboratory test before a rock job begins. The term "drillability" is used here to mean primarily the rate at which the tool penetrates into the rock, but in the wider sense it extends to the resulting hole quality, the straightness of the hole, the risk of tool jamming, etc. Tool wear is often proportional to drillability, though it also depends on how abrasive the rock is.

The drillability of a given rock is determined by a number of factors. Foremost among these are the mineral composition, the grain size and the brittleness. Attempts are often made to describe drillabil­ity by stating the compressive strength or hardness; these are crude methods but they will often suffice for rough calculations. The Norwegian Technical University has developed a more sophisticated method for calculating the DRI and BWI.

DRI (the drilling-rate index) describes how fast the drill steel can penetrate; it also includes on the one hand a measurement of brittleness and on the other hand drilling with a small rotating bit into a sample of the rock. The higher the DRI, the higher penetration rate. The DRI can vary from one rock to another.

BWI stands for "bit-wear index", and gives an indication of how fast the bit wears down. It is determined by an abrasive test. The higher the BWI, the faster will be the wear. In most cases the BWI and the DRI are inversely proportional to each other, so that a high DRI will give a low BWI and vice versa. However, the presence of hard minerals may produce heavy wear on the bit despite relati­vely good drillability. This is particularly true in the case of quartz. The quartz content has been shown to exert great influence on wear, which explains why a relatively easily drilled sandstone, for example, can cause very heavy wear on the bits. In other cases, metamorphic, compact, quartz-bearing rocks may also prove to be very difficult to drill.

So, the nature of a formation to be drilled has a great deal to do with the success of the drilling operations. Here, the assistance of petroleum geologists and information from off­set wells can give the driller valuable informa­tion regarding the type of formation he is drill­ing in, information that will affect the way he drills the well.